You've only got 3 options...
A new crankcase..
Helicoil inserts (if there's any in that thread size!)
Tapping for the next size up bolt who's tapping drill size is the same as whatever size the original bolt was (for instance, going from 3mm to 3.5mm).

Head bolts don't need to be tightened to within an inch of their life. The one advantage of the old screwdriver heads was it was near impossible to strip them unless you were a gorilla. An allen key gives a lot more leverage, too much for these small screws going into alloy. All that's needed is a reasonably firm pressure from a couple of fingers resting against the side of the allen key.