The entire stud was broken off on the driver side of my car when I got it. I turned a new one in the lathe, machined the ragged hole in the plate, and welded it back together. You can't tell it from original. If I had your problem, I would cut off the stripped part of the stud at the larger diameter, then drill and tap the stud, and insert a section of grade 8 all thread or a long headless bolt secured by red loctite and maybe a pin or tack weld if you are nervous.

You may be able to get away with running an M12 die over the stripped stub, but you better get it started straight. And that could be a tricky operation.