At the end of the day the studs are just (HT?) steel rod threaded into the barrel. If they snap off, as long as there is sufficient metal around them, they can be drilled out, the hole re-threaded oversize, and a new stud made and locked in place. A good engine machine shop will advise you, There are proprietary items such as "helicoil" inserts which may suit, or an insert can be machined. Care would have to be taken over positioning, verticality etc etc, so it's not a job for your local garage. It may also be that, if the machinist can cut the stud off flush bith the barrel and drill it carefully & incrementally, once there is only a thin "wall" of stud left attached to the barrel that it will lose strength and can be taken out with a stud extractor. Again, a good machine shop will know - someone who works on truck/digger/tractor engine . . . Good luck