If it was my motor, i would get out my dremel with  a fine tip cutting bit and cut away the weld. Starting in the center of the bead (weld face) cut a channel  almost to end of bead. Then do other side channel, don't cut all the way down to the root just yet. Slowly cut away the bead on the shaft. Cut it like your removing bad threads and keeping a cylinder profile. Now slowly cut away bead on nut. Now all that is left is the root. Cut away the root to the shaft. Repeat on other bead the spin off nut...have machine shop check Balance...if its good, buy a new nut