Here's an old school trick. Put the problem cylinder at bottom dead centre. Next take some small diameter hemp rope or equivelant and fill the cylinder with it to the base if the plug hole after soaking the rope in water. Now you have two options. Carefully drill the thing out and now you don't have to worry about cuttings going into the cylinder. If that's a no go heat the broken portion with oxy/aceteyne. Once its nice and hot use a wide blade heavy duty slotted screwdriver, chisel or eazy out with plenty of non flammable liquid wrench. Then carefully try removing the stubborn broken off plug.
You get better results if drill the bulk of the broken off piece out first then try heating/removing the leftover. It will tend to split and come apart easier.
The rope keeps crap from getting into the cylinder. Once you get the stubborn piece out, take a shop vac and put it over the hole and draw out the rope.
Don't worry about damaging the threads in the head too much, a heli-coil kit works wonders.