If it's the allen fastener that is stripped, look at some easy-outs (screw extractor sets, Speedouts, etc.). Vermont, Irwin, Speedout, Bosch, Hanson are some brands. If it is a tight application you may be able to find (or fabricate) a right angled version. If the allen isn't too damaged, I find using Torx or other allen sizes (metric and standard) can sometimes help.
If it's the threads into the engine that are stripped, the above recommendations for Heli-Coil type of repairs is pretty doable. Not sure if it is a blind hole or a thru-hole (and where that thru-hole goes). Let us know, I'm assuming it is blind for now. Hopefully it's easy to get to so you have clearance without pulling a bunch of stuff apart.
Let us know more details if you can, that way we can provide more detailed ideas for you.