how bad is it? are the heads still on? did drilling go off far enough to hit the block? did you get to drill into the bolt at all?
try a piece of steel rod, grind the end so that when rotating CCW there is a bur sticking out that may grab the bolt.
take a long allen wrench that won't quite fit the hole in the bolt. cut to make it straight, grind tip to where it has a sharp angle, (not a point) pound it into the hole cutting new flutes as it goes.
put a small chisel into the hole,,CAREFULLY smack it a couple times to create a slot, then use screwdriver to remove.
be certain to hit the same spot, you don't need 3 or 4 different small slots......use this method only if desperate, may cause the bolt to expand,,,very undesirable.
here's a good one, if you can get any kind of rod into the offset hole in the bolt,,,, then put a tap into the hole with the rod in one of the flutes of the tap. see if you can turn it that way.
if the head is off, piece of cake. all you need is a nail and small hammer.
it's too late now, but when drilling broken bolts always use a guide tube to center the drill. broken bolts have hard and soft areas, they don't drill straight.
free advice, take it for what it's worth.