IF it's broken clear enough to pull off the works provided you can get the other bolt(s) out, Sears has a set of of "bolt out" sockets just for your predicament. I would also soak the non broken one in PB Blaster or something like it overnight. If you can get to the thread side, do it, too. The next day, slightly tighten it, add a squirt more P.O. then remove.
They come in 'regular' and low profile. I've used them and they are good. "Easy out" or other drill and insert types are best avoided.
Look here