That could be a tough one. Reed valve or piston ported two stroke engines can be made to run in either direction just by changing the ignition timing. However, with a twin you could possibly run into problems with one cylinder running richer than the other if you make it run backwards to the way it was designed. If the engine had individual carbs for each cylinder, that would not be an issue.

Another issue with making them run "backwards" is that the prop hub and/or prop retaining bolt or stud needs to be left hand thread to run in reverse. Otherwise the power strokes act like an impact wrench, quickly causing the prop hub or prop to come loose. Careful with that, as the prop can come right off in a lively fashion. (Don't ask me how I know....)

AV8TOR