Engine design is optimized for the direction of rotation intended for that specific engine. Induction timing, exhaust timing and ignition timing all play a part as do a multitude of other variables. Some engines, Cox .049 comes to mind, will run backwards as good or almost as good as they do forward. The more complicated the engine, i.e. 2 stroke vs 4 stroke, the more difficult it is to make it run backwards. However, most any engine design can be reversed ON PAPER. And when built from that, will run as well as the original.