Mines a standard 530 chain so could be one of your sprockets!
Suppliers seem to sell pistons with rings but not rings on their own (least I couldn't find them) and prices are silly. I sourced my Honda rings from Japan and they cost just £19.10p inc postage.
In the service history files I was given an engine manual photocopy which seems OK. Also have a Delorto carb manual but I scrapped the carb and fitted a better type.
Buying a few engines at a time would make the serial numbering system complex, in terms of working out total amounts of bikes built etc. The rear hub on mine has an inbuilt cush drive rubber so I'm surprised the standard spokes break. It's a light bike and not pulling a chair. Must be the lacing as you say. I don't know if the army MT500 has the same issue, the rear wheels look the same?
I do have one question which is unusual and you might not know so don't worry. When I stripped the motor to service the top end I did plan on stripping the bottom end just to have a look over inside, but when I tried to undo the crank cam belt pulley it wouldn't shift, so I gave up and left it alone. Is the thread right or left handed? I might need to get the pully off one day to service the clutch if it ever wears and I need to be careful.