On a 4c you need a new cam and the timing must be changed.
On a 2c pistion port just change the timing. Do it all the time, some engines even run backwards better than forward.
ORIGINAL: HornetFitter
It is never going to work . The transfer port timing (in a 2-stroke)/ valve timing (in a 4-stroke) is different when turning in reverse.
If you could redesign the engine it might be feasable, but the performance would be just plain poor.