#1: if you can get to broken bolts easily you have a chance
#2: if you can't get to them easily, you are screwed
Be sure you smooth the surface of the broken bolt first if there is nothing to grip onto. Then center punch it and be sure to be dead center. Then, using the best quality metal-grade bit you can find like cobalt or even carbide (if you can find one), drill a hole in the bolt that is deep enough to get an easyout or left handed drill bit into. Run the easyout in reverse on the drill and the bolt should come out with it. Hopefully the easyout won't break off and leave you a bigger problem, that's happened to me before. Lubricate the bit frequently, and back it out frequently to allow the "dust" to clear out of the hole. That makes the bit last longer, but be aware that it may take 2-3 bits to drill a bolt in some cases.
This method is only feasible if you can see the broken bolt easily. If the bolt is on the bottom or side of the motor then you are in trouble because it would be damn hard to be able to get a center punch in there if you can't see it, and even harder to make sure you are drilling straight into the bit. That's, of course, if you can get a drill in there to begin with. If you've taken the motor out then it's obviously not an issue.
Keep in mind that if it is in the block you are probably trying to drill out a very hard steel bolt from much softer aluminum, so if you are off center the bit will chew into the aluminum relatively quickly compared to the steel.