the thermostat housing bolts to the head not the lower timing cover, if it's the lower cover that would be the water pump, the water pump could stand to loose a bolt or two, it just needs to be held in the bore the o-ring seals it tightly. as for the head, id drill em out, if the threads survive cool otherwise id heli coil them. the mother of doing the lower cover for the small time guy, is the 22mm crank bolt is real tight, as in 3/4 drive tools tight, the only other issue is being careful with the gaskets above and below the cover. if you need a timing cover i have several core motors, 1-866-bmw-guys 8-5:30 m-f 9-1 sat cdt
all america wants is cold beer warm cat and a place to take a poop with a door on it