Are you sure your rings are warn out to the point of replacement?.....on my old Tanaka 3 HP the rings tended to get stuck in their grooves with carbon....I would very carefully remove them, clean rings and grooves, re-install and motor would run like a champ....started using Ringfree in the gas and rings stayed free. If your piston is really warn and no replacement is available a machine shop may be able to knurl the skirt sufficient to help enlarge it....but I doubt it's warn that much...