CruizinImage pistons are fine for the street and prices are great. I have Bill Bune in Anoka MN do all my GT750 cranks. Good prices, great work and fast. No need for Wossners or Wisecos on a simple street bike IMHO.
That bike is interesting in that it appears to have a 74 tank on it and not a 76 tank which was a different shape. I have brought home worse piles of parts than that, but that will be a major project.
I would forget about reed valves. It can be done, but it's a major PIA with a lot of welding and machining. GT750 motors are a really low state of tune, so they are nice to just ride around and have fun on.
Here's my Sunday ride..
http://pinkpossum.com/GT750/phattrakka2/PTdrag.htm
It's pretty easy to get more power out of a GT750. 80 is easy, but porting them is a PIA. I have done a few of them to different stages of tune and it's easy to forget how much time and work is involved in getting all 3 the same. The simple approach is to raise the barrel by slipping a lift plate (spacer) under the barrels and machine the same amount off the top of the barrels. people will tell you that you cannot run stock CV carbs without a stock airbox, and they are just wrong. I ran mine with velocity stacks, UNI pods, and drag race short intakes and they run just fine with a few jetting changes.
Chambers make the bike run lean, so don't forget to jet up if you use them.
To run on the track, a stock frame is heavy and not very stiff. They can be lightened and stiffened, but a CMR frame is the way to go, but pricing is a way above my pay grade.
For ignition, the simplest is probably Accent from Germany. For head and base gaskets, use COMETIC on a hot motor. Aftermarket head gaskets are all too large and will leak on a high compression motor, but are fine on a street motor.
Have fun with it. I have 4 GT750s in the garage right now. The Phat Trakka, a Dunstall, a radical restomod and a cafe racer which is on teh frame jig and has modern suspension. I have restored a couple and getting all the correct parts is a fun chase around the world, but gets to be very expensive very fast. I prefer non standard bikes and build them for me, so I get to use whatever parts I want.