If the inner hex is deformed so the wrench can't turn the screw...
1) if the screw head is accessable, grind a slot in it with a Dremel Cut-off wheel, then use a straight blade screwdriver.
2) A left-hand tap will often work well as a screw remover.
3) the "Torx" bts will somnetimes grip in a hex key screw when the correct hex key won't turn the screw.

If the screw is stripped in a manner that turning it doesn't remove it...
Gently insert a straight blade screwdriver between the head and the engine cylinder case. Twist or pry with gentle pressure as you turn the screw.
Whent he screw is out... hopefully (rarely happens [img]i/expressions/face-icon-small-sad.gif[/img] ) the screw is stripped not the case threads. A stripped screw is easy to replace.
If you have to re-tap the case, you have to go to the next larger (sometimes 2 sizes larger...) screw dia. You'll also have to drill the head's hole for that screw to accept the larger screw, and you may have to grind the fins and redo the flat for the screw head to seat properly on the engine head.