The fairly standard repair for aluminum cylinder heads with stripped plug threads is to install a Helicoil. The existing (stripped) hole is tapped for a larger thread, and then a threaded insert is installed using a somewhat special tool. The insert has an internal thread which accommodates the spark plug.

All things being equal, it is *best* to do this with the head off the car to avoid metal chips falling into the cylinder, but in reality few people remove the head for this repair. One standard trick is to load up the flutes of the tap with heavy grease so the chips get caught; there may be others.

If you're talented and know what you're doing, then get the IPD kit; I am both (depending on who you talk to) and I've also installed oodles of Helicoils from a #2 thread size up to 1 inch, but I would turn this over to a pro (so if *he* mucks it up, he gets to fix it for real).