Never seen any OS engine, much less their 60s, backfire more than once. Something does not make sense about the backfiring. Two cycles very seldom backfire. As for them running backwards........ Their timing is VERY solidly set. No way to vary it unless you've disassembled it and reassembled it with the sleeve in backward.

Solve the backfiring. You have something very wrong with the engine or the way you're handling it.