Depending on how deep it is in there id get a screw extractor and as carfully as you can, and with the sharpest bit you can get for that extractor drill perfectly in the center of the bolt and extract that out. You could also get a drill bit long enough and if the bolt isnt "frozen" in the block you can put your drill in reverse and push on the bolt as hard as you can and it might back the bolt out. Its worked for me in the past, not on a head bolt but on other things it has worked.