I've had good luck with welding something to the end of the broken bolt. Good option is to find a nut that fits over the bolt then weld in the hole, fusing the bolt to the nut. Often, the extreme heat shock of welding will help break the bolt loose. I did one stuck diff. fill plug like that and after welding on a nut, it almost came out by hand.

I imagine oxy welding might work, have never tried it for that purpose. You'll need a pretty good size torch to heat things up enough to weld. Might also try brazing the nut on.