Welcome Glenn,
Getting piston rings shouldn't be too much of a problem, but first, can you give us your approximate location on the planet, as then we won't end up recommending parts suppliers who may be on the opposite side of the globe to you?
Piston rings normally come in sets of three = Two upper "compression" rings (which may or may not be exactly identical) and a third "oil scraper" (or "oil control") ring.
You will need to order either standard or whatever oversize rings are required to match the piston size, so look for any markings on the tops of the pistons to see if they are marked "STD" (standard), or if the cylinders have already been re bored and oversize pistons fitted they should be marked either +.020, +.030, +.040, etc.. The standard Domi 99 (600cc) bore size was 68mm, and +.020 will be twenty thousandths of an inch larger, and +.030 is thirty thousandths larger, etc.
If you cannot find any markings on the pistons, then you will need to accurately measure the cylinder bores to determine if they are still standard 68mm size or not?
If the bores are worn, then they may need re-boring anyway, so then you'd need new oversize pistons and rings to match the over size.
You could try cleaning up the clutch plates, but again there shouldn't be any problem getting new ones if necessary.
