what size big bore? You can drop the 60cc YT piston in there but you have to offset bore the cylinder to make it work. if you had someone not knowledgeable in this you might not get a good seal at the base of the cylinder...and have an air leak.
spray carb cleaner at the base of the cylinder while running and see if the idle changes.
But a run on situation is usually a leak at the carb or intake. so do the same carb cleaner trick up there. my guess is a leak at the carb to intake bushing. PITA on older pw's.