You can try a 'counterclockwise' drill bit. Don't know if this is the proper english name for it, but basically it is a drill which cuts the other way around.
When it cuts into your screw it grabs it and unwinds it.
Most of the more advanced hardware stores will carry this kind of drillbit.
The best solution is a counter-clockwise tap, but the screw is properly too small for it and it is kind of expensive if you have to buy a set.

But I think you could still run the engine with 3 screws without a problem, I ran an engine with 2 screws in the backplate for a day because the other two were to damaged to use them. The O-ring makes the engine air-tight, the backplate won't buckle or anything because it misses one screw.
You could buy some 'lucky 7' engine sealant to be 100% sure, this has been made especially to use on a assembled engine.