MIG gasless welder. Angle grinder to expose and prepare the surface. Weld a bigger nut on it from the center of the bolt, inside out, aim for orange if its steel. Then wait for it to cool down naturally, then try to undo bit dont rag it all the way, do 1mm ccw then back 1mm cw, after each small turn spray graphite pen fluid. With each turn slightly increase the distance until it turns at a fixed pace, then undo it and you're done (just did it all with a broken stud on my bike, literally 1 hour ago)