I recently had occasion to repair stripped cylinder head threads for 1/4-32 spark plug (or glo plug--same thread).
The repair kit is RECOIL #34040-32 and it contains all tools and supplies necessary: drill, tap, insert tool and a dozen "heli-coil" inserts.
I practiced on a scrap piece of aluminum, but practice wasn't necessary. Everything worked exactly as desired.
For engines still in production, it's easier to order a new cylinder head, but for others.....
Floyd