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Lots of ways to do it, the calliper has to fit the disc width and since your disc is fixed on the hub and has no way to float on that hub, then you need to have it aligned perfect if not float. 4-pot calliper or 2-pot opposing piston type calliper design are nice in that they hydraulically self centre and far out perform any brake that only works on the one side.

Aluminum is a far more appropriate and easier material to work with when you are making brake mounts that are bolted to aluminum fork legs and the lighter your unsung weight the less mass your shock absorbers have to deal with. Think Aluminum.
 

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Yep, the early hinge pin with round pads was fraught with problems, and no, pistons on both sides do not require additional hoses, the plumbing is done internal to the brake. Your disc is dish shaped to distance it from the spokes and make clearance for the calliper, your new calliper will need to clear the spokes similarly.
You can flip your forks around to make the calliper mount on the trailing edge of the forks instead of a leading link calliper mount, that modification has some theoretical advantages both in brake force direction and reduces stress on the mounting points which become compressive instead of tensile.
One of the best mount points for a brake bracket is the axle itself, if your bracket can fit where a spacer currently sits on the axle you have an excellent place to secure your bracket.
axle mount example shown for clarity:
 

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I'd be taking a walk through the local bike salvage yard to see what you can find in the way of calliper options, your bracket design is going to be dependent on the parts you can source. The old honda design only hinged like that because only one puck physically moved and the other one had to move as it wore down to stay centred on the disc. Other single side designs use slides or sliding bolts, opposing pistons don't need to have slides, only the brake pads move.
 

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The photo does not enlarge for me but from what I can see that bracket is very long and they went out of their way to affix it way high up on the fork leg where the mounting points already existed. It is the axle and wheel bearing that you are trying to stop from rotating so the closer your mounting bracket is to the axle the less leverage that arm will have to shear the bolts that are holding it or fracture the fork leg. I think you could design a better one then that, but it goes to show you there are a number of different ways to achieve it.

The two pistons on the same side significantly increases the hydraulic pressure and brake pad friction area, but that calliper assembly still needs to move sideways on slide bolts as the pads wear to keep the calliper centred on the disc and that type of brake does not provide the feedback at the lever an opposing piston brake excels at. It does provide slightly greater clearance from the spokes to have a static pad on the calliper inside, but there are a lot of motorcycles out there with 4-pot brakes that have made it work so the clearance problem is rarely insurmountable. You are effectively upgrading from first generation disc brakes to about third generation disc brakes when the state of current technology is more like 5th. generation disc brakes. Still a significant upgrade that will result in far better braking then what you had.

The complication with swapping the left fork leg and the right fork leg is that your speedometer drive will no longer work and your fender bracket supports would be in the wrong place, otherwise that would be a better solution to reposition the calliper trailing to the fork leg.
 

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As a side note to clearance from your wheel and spokes, when they draw diagrams of how opposing piston brake callipers compare to floating calliper designs they generally over-look one important point.


In the opposing piston brake calliper each of the pistons only need to travel half the distance which means the calliper construction is Not twice as wide. In reality fixed disc callipers are not much wider then the floating version.
 

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No worries on brake pad wear, if your pads are not wearing evenly, you have a major problem.

Doesn't matter if it's a race bike or minibike, the brakes have got to work and be totally reliable, you can't ride good if your equipment is lacking and in this case there is no reason for it to be lacking, you are working on an antique that sold for about 600$ when they were brand new, complete with a factory warranty. The cable disc brake was one of the major reasons I passed on buying one new, I wouldn't want to ride an authentic one but a modified one with some suspension and tire improvements might be a blast.
 

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His contact patch kinda sucks on that humongous disc and it's been drilled weird, the only thing going for those 2-pot floating callipers is cheap and plentiful in the used market.

FWIW: a 4-pot calliper new would cost about 3 or 400$ plus the hose ~ 150$, plus another 400$ for the brake master cylinder and lever assembly with switch electrics for the rear brake light and the lever. Basically you are looking to bring it in at under ~1000$ worth of new parts or you might better just buy new parts.
 

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No reason you can't machine the original disc to make it a completely different dimension or even turn it into a floating disc carrier, there's plenty of metal there to work with.

Alignment of the disc and calliper is critical particularly if the disc can't float and that whole brake assemblies performance is dependent on your wheel bearings being perfect.
 

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I have a brand new 4-pot in my hand right now, it measures one inch from the centre of the disc, the whole brake is just 2 inches wide. But it is made for a thin profile disc, similar to the 2-pot in your photo the heigh of the pad is not very tall.
Enduro and MX 2-pot brakes have the slide bolts built right into them, you don't need to add it, they already float.

1 finger brakes are great! I don't need more then one finger to stop my MV Agusta, it has real brakes just like a trials bike :cool: it's a little tricky braking right now because our wet roads are still covered with winter sand and salt.
 

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The 4-pot I have as spare is a Beta brake, it was manufactured by AJP and AJP now makes the same brake under the name Braktec. The newest versions are monoblock designed, although personally I prefer the earlier model that is bolted together because they are way easier to service then the mono block. The 4-pots ride on the front of the fork leg by convention but not because of a design requirement.

Google up "Braktec.com" they are the major producer of hydraulic brake parts for dirt bikes. They make both the callipers and the master cylinder lever sets as well as rear brake sets. The same brake is used all almost all trials bikes made including Montesa, TRS, Sherco, Beta, Gasgas and Vertigo for the last 20 years. The 2-pot Braktec makes for enduro and MX bikes will fit on nearly all of the euro enduro bikes including KTM, Husqvarna, Gasgas, Beta and more.

If I was going to offer something as a kit, I'd be either replacing or turning the original disc down to fit the brake. A bracket would not be too difficult for the 4-pot that mounts with only 2 bolts, the 2-pot enduro bikes use 3 mount bolts and have 2 different bolt patterns, the 2-P brake normally fits on the trailing side of a USD fork.

Given a choice between a one finger front brake and a brake that is inadequate to be safe, I'll go for the one finger brake because I learned how to not slide the front tire or accidentally nose wheelie a very long time ago.
 

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Man I just took a peak at braktec, looks like really nice high quality equipment, especially the reservoir levers. But with that comes high prices. Add a new disk to it (which the right thing to do would be to take a piece of stainless steel round stock, probably 10-12" diameter, and have it lathed and milled to a precise shape that fits the original mounting bolts) and you would have a front brake unmatched by any vintage rebuild. But just the price of stainless steel... Oooo sends shivers down my spine 😱

999 out of 1000 people are gonna look for an economy option. But I see you have fine taste for motorcycles, way outta my budget lol
Yep, world class state of the art technology. I have expensive taste in motorcycles, I like them made of titanium and designed to burn race fuel, and everything has to work like my life depends on it because it usually does end up depending on it. :D Perfect day riding here today I was doing some rider training, still no bugs yet and traction was excellent. Love this time of year :cool:
 
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