You will have to remove the head and have what is called a Helicoil installed in the plug hole. (search this site for "helicoil" for more info)
Auto,motorcycle, and mower repair shops can install the helicoil for you.
Stay away from any "liquid repairs", they don't work.
another suggestion is to get another head. Always screw in the spark plug by hand until it seats, then snug with wrench.