Is there anything sticking out you can get a vice grip on?
Do you know anyone with an electric welder (Stick.mig/tig)
Hold a nut over the broken piece and weld through to broken bolt, allow to cool then unscrew
be really careful if you have to drill it I've seen several with 'scrap' cases when drill wandered off center or 'easy out' snaps in there (don't use easy out, they are crap)