spend the money have a pro, ( see i said a pro mechanic ) ASE Master certified technician remove the cyl head and replace it or send to a machine shop and repair the spark plug hole.
have it done right the first time you'll spend less money.
Taking out the head cover is really quite simple if you know how. If the head gasket is worn, you'd really want to replace that in the process as well. You'll need to put some RTV Sealant in the right spots to prevent oil leaks. I suggest that you check out a service manual. However, I think you'd really want to get the rethreading done right, else you may have a spark plug stuck in position... which is not good. But then, I think $500 is way too expensive. (Though I don't really know the cost of labour in the US.) It may be cheaper to go to a junkyard and get a replacement head cover.. or get another mechanic.
For info of anybody with this problem, repair in situ is an absolute no no. The process consists of drilling out and tapping the hole to suit a diamond section wire that is wound in to bring the hole back to the spark plug thread size. If done in situ swarf from the drilling and tapping can drop onto the piston and when fired up could damage the valves and valve seats, the other possibility is if the wire is wound in to far and sticks out into the combustion space it could become a hot pre ignition point.
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