Front end swap, chicken and egg....

[TrialsRider]

...
The original Benelli axle is an aluminium hollow tube with a diameter much larger than the original KZ1300 axle, which is (only) a solid metal bar, but obviously up to the task of dealing with the KZ's power and weight. ....

Sorry but No it's not!
Those forks require the rigidity of that large diameter axle to keep the fork legs from going ups and down independently, but you will find out.
carriage bolts connected to helicopter shafts

[McBoney]

Ok, thank you for the advice.

I had already decided not to put the original wheel back in and have now found two options. One a wheel off a BWM 1100gs and another off a Kawasaki GS 1000 and both have significantly thicker spindles.

I shall investigate what size these are and pick the most suitable.

[TrialsRider]

... One a wheel off a BWM 1100gs and another off a Kawasaki GS 1000 and both have significantly thicker spindles. ....

The front axle off the modern BMW is likely about a 200$ one inch diameter, precisely machined length of titanium,
while that is off a mono-shock front end bike and not one fitted with USD forks it's still definitely a major upgrade from the cheesy half inch steel carriage bolt and spacers concept

[McBoney]

I prefer the BMW wheel, but the original axle is only 20mm thick. The Benelli spindle holes are 25mm (screw end) and 30mm (clamp end).

But it is the axles are the same length (so I can also use the Benelli lower yoke).

Is the BMW 20mm original axle enough? If I use the original axle, I need to somehow do something at the fork spindle ends to make those fit a small dia axle... which is of course possible, but I won't bother if a 20mm dia axle is not deemed strong enough...

[TrialsRider]

I prefer the BMW wheel, but the original axle is only 20mm thick. The Benelli spindle holes are 25mm (screw end) and 30mm (clamp end).

But it is the axles are the same length (so I can also use the Benelli lower yoke).

Is the BMW 20mm original axle enough? If I use the original axle, I need to somehow do something at the fork spindle ends to make those fit a small dia axle... which is of course possible, but I won't bother if a 20mm dia axle is not deemed strong enough...

Your front forks are Not something you want to ever have fail catastrophically. I don't want to see you hurt, so I'm going with the safe answer which is; you should not mix and match front suspension components that were never designed to operate together like that or catastrophic failure is on you and not the manufacturer.

[McBoney]

You will all be glad to know I have managed to secure an original wheel and spindle to the forks, as well as brembo brake callipers, all from the Benelli Tre K Amazonas. It is a nice enough wheel, the right size, and the fact that the spindle and brakes etc. simply bolt on means so much less work, and as mentioned here, probably the safest.

Now the entire front end comes from one bike... lets see if the back end can keep up! ;-)

Paul

[McBoney]

Setback: the wheel appeared to have a small dent in it and when I took the tyre off for a specialist to look at maybe repairing that (some small dents can be repaired by specialists), he immediately noticed a crack as well. I hadn't seen the crack as it was so small, but after rubbing some yellow crayon on the area, it was clearly visible

So lesson learnt: Check second hand items VERY carefully and preferably get a specialist to look at them.

Wheel is scrap...

[TrialsRider]

Not surprised, seen lots of rims like that & way too many people will sell absolutely anything just to make a few bucks.