Working in my diesel shop we used remove broken bolts with head in place , put a piece of black pipe into the headbolt hole and weld the inside of the pipe to the top of the broken piece. The heat helps break the threads free, you leave about 4 inches above the head and use a pipe wrench to rotate the pipe . Did this several times and it works, I believe we used stainless rod so the weld would not stick to the cast iron. Be aware that if it doesn't come out drilling it later will be more difficult.I have also tried using the broken piece to twist it out like it was mentioned above it may come out easy. Good luck
