as above it does depend where its snapped.. if its deep down in the block (which i dont imagine it will be as there would be no stress there then your fucked other wise if its at the top.. or if there is a bit protruding then it will be simple as. The head bolt itself wont be 'tight' in the block and should unscrew easily.. the only thing it torques down to is the head itself.
If it was me i would just cut a notch in the top with a hacksaw and unscrew it!
For gods sake cover the bores up when your doing this as you wont want swarf going into the bores