Originally Posted by DS Motorsport
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.
Yes, a counter clockwise, aka left hand drill bit, spun by a drill motor (backward of course)
Heat the crank case up to around 200-220 to expand the aluminum to make the steel screw easier to remove. also use a penetrating lubricant,
Drill into the remaining part of the screw, with fresh new drillbit, apply pressure to drill hopefully the bit will "bite" into the remainder of the screw and back it out.
This can be tricky if the screw has broken off at a weird angle (not very flat) as it may cause the bit to wander and drill into the crankcase threads. Turn the drill motor slowly.
good luck! heat is your friend!