Can you remove the head? If so that might expose the remaining shank of the broken bolt. If there is enough to grip with a pair of Vice Grips, then heat the surrounding area with a Monokote heat gun, or very light flame and when the cylinder case is hot try to remove the bolt by turning the Vice Grip pliers.

If the shank is busted below the surface of the head there are three options that I am aware of. #1 would be drill out the broken bolt, about 95% worth and then using a dental pick scrape out the remaining slivers of thread. #2 would be drill out the bolt 100% and then re-cut new threads for a larger bolt. #3 would be send it to a machine shop that has EDM capabilities and have them EDM out the broken bolt ($$$).

http://www.cncmachining.org/machine-shop/edm.html