The process is to drill into the center of the broken bolt and then hammer a splined piece into the hole and remove the screw. This is MUCH harder than it sounds. You need to apply heat and rust penatrant. You also have to drill a hard erratic surface. You probably should pull the engine to get it done.
For a start put some rust penetrant oil on the bolt holes to help start loosening them for a day or so before anyone works on it.
Good luck, it's not easy for a problem that happens so quickly.