The studs are screwed in. Cylinder studs are usually tightened to only about 10 pound feet of torque.. 10 pound feet is almost nothin.. any more and they snap easily. So, you should always use a small inch-pound torque wrench or risk breaking them again..
I heard a good trick to remove one thats broken off flush, instead of drilling them out and using those stud-remover things...
Position a small nut onto whatever broken stud end is still sticking out and weld the inside of the nut onto the broken stud's end. Then just use a wrench and unscrew it. New studs should be easy to find.. a good hardware store might have a bolt or something that can be adapted.
Running with broken or loose cylinder studs or with a head gasket leak is like begging to destroy a few things. There's a good chance that at least the head is now warped. So, everything should be disassembled and carefully examined and repaired before you run it again.