Thank you for your answers,
It seems that the only thing needed in order to make the engine work currectly (but opposite direction) is to re-time it.

What about cooling?
in a pusher mode, the prop pulles air from the cylinder head (instead of pushing air into the cooling fins), could this be a problem?
the modified engine will be mounted as a pusher without any option for baffeling or jacketing,
further more, the plane (actually - VTOL) will spend alot of time hovering and will not have free air flow caused by straight flight.