A little out of the box, buy I have used an automotive spark plug thread repair kit in the past. Available at local big box auto stores. It comes with a combination drill and tap tool (so you don't have to disassemble the gearbox, just put a lot of grease on the tool as you use it to catch the chips) and several steel inserts. You will have to by the correct thread size drain plug and copper gasket as well.
They are made to repair stripped out spark plug holes without removing the head. The thread insert is steel and has teeth on it to lock into the aluminum case to keep it from backing out.
I repaired several stripped drain plugs in various things this way. End of problem. If I remember right the kit is about $45.