The mower is probably not worth more than $250 or so. I don't think it will be worth the trouble to tear the engine apart to replace the governor parts. The thought crossed my mind that I could loosen the throttle linkage so that it's not affected by the governor, and that would probably be OK for basic use, but I don't want to sell it that way.
Sounds like the governor linkage may be stuck, broken linkage/spring. Look for dirt/debri around the governor linkage and wash/brush/blow any debri from linkage and ensure linkage mechanism is intact and hooked up and throttle cable is still hooked up and not broken/seized on. Links below may be of some assistance. Good luck.