One trick that I have used sucessfully in cases like this is to use a die grinder (or Dremel) with a small cutoff wheel (dremel style 1" dia.) to cut a slot in the broken top of the bolt being careful not to nick the engine block. Then use a flat screwdriver in the slot. I used this trick to remove a broken valve cover bolt on my Miata. Add a bunch of penetrating oil to help smooth things along. If the top is broken at an angle and you need to drill out the bolt, you can use a small end mill in a die grinder to level things out. Always center punch before you drill!
