You could try out a tool that you insert into the spark plug hole , it expands and you remove the tool counter clockwise and it will chase the original threads. This tool works good because the shavings are removed with the tool backwards and leaves no shavings in the internals of the engine (which is bad).It might work enough to keep from making major repairs. Otherwise you will need to drill out the hole and heli-coil it, which is a lot of work and will require the head to be pulled off. that tool is really cheap at Napa auto parts but I've seen it at other places too.