I have a extractor kit from Snap-On that I use at work all the time for similar problems. It works great, has a few different left handed drill bits, and different size extractors. If you can't grab a hold of it with pliers when you get the cover off your gonna have to drill the center out, and use a extractor to back it out.....be careful bud, and good luck !!! Broken bolts suck, and in my profession they ruin my day all the time At least it's not cross threaded in there though. Usually a steel bolt in a aluminum block comes out with little effort since the head of the bolt is what provides the clamping force. It's kinda surprising that the threads didn't just strip out instead of it breaking.

 

As a machinist in the marine corps i would suggest u use the left handed drill bits. since those bolts dont have too much torque on them it should back it out. just make sure u drill it in the center, and start of small and work ur way bigger. then if the left handed bits dont work u can get a extractor, auto zone has them also.