maybe best to drain and flush the gearbox first? see if any metal parts drop out.. then refill with a good quality gear oil... when was the last time you changed it?

or maybe thats not a good idea and try to source a used gearbox or parts to rebuild it

heres an example on the price of parts, keep in mind it is in AUD.

https://www.********.au/store/advanced_search_result.php?keywords=mc21+gear&osCsid=def82e915ab80bbf0d572ec0e45e29ab&x=0&y=0

cheers