As long as a part of the bolt is still sticking out, you should be able to get the damned thing out. You need to unscrew (or break
) the other bolts first and remove the thermostat house.
When the method described above does not work, you can try welding a bolt head on the part of the broken bolt that sticks out (presuming it does stick out). The bolt (and surroundings) will get warm of the welding and can normally be screwed out...
This is just one of the downsides of driving an oldie...
greetingz,