broken bolt extraction

Have you tried left hand drills?

One of the best investments I made tool wise was lefthanded drillbits from slapon, about $30 for a set of 6, works great.Start with a small bit and then go up in size if the 1st one doesnt get it. If that doesnt work and there is enuf thread showing, weld a nut onto the threads and then try backing her out. Good luck!