It looks like there is finally a resource for oversized pistons for the KZ750 twin. These seem like a high quality option for oversized pistons, although not very cheap. You can try to round up some NOS parts that will likely be $350-450 if you can find them (NOS oversized pistons and rings are super rare!), so you might as well get this set and get a modern 3 piece oil ring instead of the stupid one piece stock oil ring. FWIW i have no affiliation with this company but i may be using them for an engine build in the future.