Unfortanitly, 50 over is the max for that Yamaha block (I know there are some that say they can be "resleaved") do as UM said and hone the cylinder and look for "score" marks in the cylinder and on the piston skirt's. Yamaha dealers have the advantage to put a new set of rings in, seat them and check for ring end gap. I like true Yamaha engine rebuild parts,because I'm a dealer,but if you want to save some money, I'd call or look at Tom @
www.cartpartsplus.com website. He carries alot of stuff and he is more then a straight shooter on price/delivery and service. He also has a primary clutch puller tool for sale.