Helicoil. m6x1 if it is one of the valve cover or cam chain inspection cover bolts. Helicoil kits run around $30 for the tap, inserts and insertion tool, and you will also need a drill, a bit,a tap handle and a tap that matches the final thread pitch of the bolt to clean it up. The stuff is expensive because they know they have you, I mean it is cheaper then a new block or whatever, right?
If you were local I would loan you all the stuff I bought to do the 1 bolt I stripped there, of course they only sell the inserts in a 10 pack.....