I took the engine to Hobby House in Reseda. A guy by the name of Jay (store owner, maybe?) had a look at it. He just popped the backplate off and noticed that it had been knocked back against the plate. There was some scratching against the inside backplate (from both of us trying to turn the crankshaft) but he said it seemed fine and there should be no problem with it. It turns again like new, so it saved me a repair bill or a replaced engine. For the help he gave me, I ended up buying $37 of stuff in his shop.