First i would make sure that the threads are really damaged. Examine the threads
visually - you are likely to find that the sparkplug was not tightened and
it's way out. There may be no thread damage at all (or very little).
If you do go the path of tapping new threads, coat the tap with something
like thick grease. This will catch the chips generated. A general rule of
to turn the tap one half to three quarters of a turn, then to back off the tap
(reverse about one quarter turn) to break the chips. In your case I recommend
you fully retract the tap and remove the chips from the grease. It is slower,