Welcome to the HQ. Just one question. Are you missing a cylinder head or are you wanting to replace your cylinder head? Either way, the head needs to be machined to correct the quench band.

If I'm not mistaken, Pro Design makes the cool head for PTR so they are the same. The size of the dome will be one of the determining factors of your compression ratio. Your compression ratio is one of your determining factors of your fuel octane requirements. And pump gas does not fill the requirements. You are gonna need at minimum, 100 octane race gas for a zilla.

I would steer away from the PWK carburetors and use a Mikuni TM-38. This can be bored out to a 41 if need be.

As for clutches, Barnett is probably one of the best.