Under-cutting gears is not a fix, it's a modification. Finding, and repairing the cause is the only fix.
Consider, they didn't come that way, so they must work. True, 500s have some known issues with transmission failures, but, it stemmed more from improper operation against a poorly engineered part. There should be numerous threads on here to help you identify with decent certainty, what is actually wrong and how to fix it. Regrettably, that may entail pulling the motor and splitting the cases. Perhaps, if that is the necessary remedy, you can source a replacement 500 or 550 engine and install it, while yours is either repaired over the winter or sold to the salvage yard to offset the replacement?
A 550 motor will essentially pop in with very little changes at all.