A broken-off EZ out is a real pain. You can heat it cherry red with a torch, and let it cool slowly, and it will soften enough to be drilled out, particularly if you use a solid carbide drill bit. The trick is to be sure you drill only the EZ out and the surrounding screw, not the block material. I'd suggest pulling the engine, and stand the block up under the spindle of a slow-speed drill press. Trying to keep doing the repair in place with hand tools is a good way to ruin the block for good.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!