grab ya self a welder,( i have the mig set to 3/4 power) zap the centre of the broken stud, keep zapping (spot welding onto the weld)to make the end of the weld come out from the head,(your basically trying to make a new stud out of mig wire ) find a large nut and hold it over the slug of weld with some long nose pliers, then continue pulling the weld out from the head(yes, you will have to aim inside the nut), it will eventually weld to the nut, give the nut a couple of taps with a spanner/hammer or neighbours cat, let it cool down or spray something on it if your impatient like me...
then slowly ease the stud out by undoing it, you might have to work it back and forwards a bit like tapping a thread or your mum...
you might break a few nuts off but itll come out eventually, just keep putting nuts on and working the bitch..
generally if you arent waving the torch all over the show you wont do any damage to the head regardless if its cast iron or alloy... also works on flange studs...
enjoy...
could be usefull in tech archives if anyone thinks its relevant..
peace.