Snap-on and K & D, and likely other manufacturers make a thread chaser that expands. The expansion mechanism works similar to the locking mechanism of a bicycle handlebar.
In the contracted state the diameter is smaller than the spark plug hole. You insert the tool into the bunged up hole, expand it at the bottom where there are still good threads and back the tool out of the spark plug hole. I have been fortunate and never had to try this tool, but to me it seems like the best chance for restoring the existing threads. Call your mechanic and see if he has one?
Dan
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