I've had the same problem, am restoring a vintage Boomerang and have lost one C-ring, after reading many posts here I've a few options, but don't want to fork out for a re-re gearbox if I don't need to. (Went down the e-ring route initially and could modify a bevel gear but would rather not if possible. A c-ring or 2 would be amazing if anyone can spare.)
(Tested my controller/radio and servos today, still working after 30 years in my parents loft )
Cheers!