here's a possible solution.
there looks to be plenty of metal so you could drill to the next larger size, and then using the proper loctite, red i think, the stuff that locks threads permanently, then using either a heli coil, or a brass bushing-threaded i.d. and o.d., you could use a different plug. one that's readily available. i'm leaning towards going to the harley shop and getting a drain plug with the magnet in it. transmission and engine both use a good drain plug that has a magnet in it. i'll update this reply when i get to work on changing my differential oil. hopefully i'll come up with a suitable solution. a replacement drain plug with a magnet would be best it seems. something that's readily available and not made out of unobtanium like triumph parts.