I don't think you're considering crankshaft rotation. By mounting the engine backwards in the frame you'd be driving the rear wheel backwards. It can be done but you'd need another jackshaft to change the rotation which defeats your purpose.
May I ask why you'd want to do this? I assume you want to use your existing gears without resorting to a shift kit and the inherent cost and complexity. One of the biggest hurdles you'd have to overcome is the design of the chain drive and sprocket location.
We're also assuming you're talking about a Chinese 2 stroke engine. If you're looking at a four stroke, you'll be faced with basically the same obstacles.
Tom