You'll have to drill out the hole and tap new threads. Then install a timesert spark plug insert. Do NOT use an ordinary helicoil. You hook up a shop vac so it's blowing into the tailpipe and rotate the engine by hand until you feel air coming out the spark plug hole (exhaust valve open). Then you drill so all the chips get blown out. Coat the tap with grease and do the same thing. Then screw in the insert.