look closely at a new gear. I think there is a definite offset of the teeth in respect to the hub. Puttin one on 'backwards' will definitely cause serious misalignment and rapid self destruction. There is also usually (no sure on your engine) a thick (3MM+/-) thrust washer between the sprocket and fron cam tower. Be sure and replace the chain and the gear and any worn guides., and give yourself a big congratulations for searching for the problem before it trashed the engine.