I have had a similar situation in the past except that the bolt was broken off in the engine case. You can get a bolt extractor at most hardware stores. I got mine at Sears. It resembles a drill bit but it goes in a tap handle. It is tapered and has very coarse reverse threads. To use one of these you need to first drill into the broken bolt with a drill bit that is small enough that it won't damage the threads. Then you screw the extractor into the drilled hole. Remember, it has reverse threads so while you are "screwing" it in, it is twisting the broken bolt out. The more you can screw it in, the more pressure on the broken bolt.