Hard to understand where you are saying the drill bit broke. If it is just inside the allen head smash the drill bit to pieces with a hammer and a sharp punch (WEAR SAFETY GLASSES). Then go back to drilling. You only want to remove the allen head. I have gotten lazy these days and just whack the bolt heads off with my plasma cutter.
Well if you drilled the head of the allen off you still have a pretty good part of the bolt exposed after you remove the manifold right? Heat is your friend, as in torch, along with smacking the end of it with a hammer. Then you can use a stud extractor or simply vise grips to remove the remaining part.