Unless the head is recessed, using vyce grips or cutting a slot as already suggested should work.
But if all else fails, choose a hex nut where the hole is almost the same size as the head of the damaged screw and place it on top of the head.
Take a wire feed (MIG) welder and aim into the hole in the nut being sure to hit the screw head first rather than the nut and fill the hole in the nut with weld.
Hopefully the nut will bond strongly enough to remove the screw.
The idea is that the nut should protect the bike from damage by the welding.
This is a "last resort" suggestion - please do not hold me responsible for any damage that does occur!