Go to NAPA...
and get yourself a spark plug rethreading kit. The NAPA inserts are not like a Helicoil, they are more like a smaller version of a plumbing bushing that reduces the thread size.
You will get a tapered tap in the set to rethread the hole with larger threads. There will also be several lengths of threaded inserts, match up one to the plug for the correct length.
Put the stripped cylinder at TDC (piston up), then turn it either direction about a quarter turn on the balancer. This is to allow room above the piston for the tapping tool, so as not to gouge the piston crown.
I use a socket and extension to get the insert started at the same angle as the other plugs are, do yourself a favor and electrical-tape the insert and socket to the extension so they do not come apart.
Tap the hole until the tool can be turned easily with the fingers. This ensures the hole is threaded the whole length for the insert. I usually do not worry too much about the aluminum shavings, they can be blown out with an air gun wand inserted in the hole. Alternately you can coat the tap with grease to catch the chips. Any remaining chips in the cylinder will easily find their way out the exhaust port when the motor fires.
Yeah, some people will complain that shavings have no place in the motor, or that you must pull the head to fix the hole, but as long as the tap is lined up correctly you can fix this in about 15 minutes on an otherwise running car with NO ill effects. I had a lot of happy customers after saving them labor to pull the head... and everybody's cars are still OK...
Now that the hole is tapped, shoot a little brake parts cleaner in the hole to be sure the threads are clean. Oil the plug threads, screw the insert on the plug hand-tight, then wipe the external insert threads with a rag and brake parts cleaner, then put some hi-temp threadlocker (in the kit) on the insert threads. Just a drop or 2, more is not better. Screw it in the hole and just snug it down with the appropriate spark plug socket, no need to be He-Man here.
When the insert is installed the threadlocker will bond it to the head, and the plug will easily come out of the insert