Honda used a one piece oil ring for a number of years before going 3 piece and then to thinner rings. Thus you can, if you use those pistons again, upgrade the rings to 3 piece oil.
On the source of the pistons: in the 70s, there were many suppliers of speed parts for SOHC Honda. RC Engineering, Powroll, Yoshimura, Wiseco and others I have forgotten.
When they developed big bore kits there were two routes to go for pistons. Forged with their own design or sand cast from the same Japanese contractors Honda used. My vague recollection is that Yoshimura got pistons the latter route, thus able to tout their parts as having the same reliability and quality as Honda's. But they got them with an unmachined dome so they could develop their own compression ratio (obviously higher than the stock 9.0 ). Similarly they, or a machine shop, could machine the wrist pin area for Teflon buttons. A shop like Yoshimura would also be using a higher lift cam (they liked high RPMs!) hence they would develop the piston kit in conjunction with the 500/550 head and the intended camshaft.
That is my guess - nothing more - as to why the domes are a bit higher. Also the 500 combustion chamber is of course different than the 750 so what results for a compression ratio in one motor will not be the same in another.
Question: what on earth do you do for a head gasket in this motor ??
Crankshaft and bearings: unless the motor was abused with dirty oil or lack of oil you should find the bearings and crank in pretty good shape. Plastigauge checking might be a good place to start.
