Oh that's not good, depends where it's snapped, if it's snapped flush to the engine block then it's drill the bolt shaft and helicoil , not easy as they are toughened bolts.
if its sheared not flush then a stud extractor could still save the day. Either way, drilling with the head off the engine with lots of metal swarf is not good with an opened up engine
not a job to be rushed
Or there is a process called Spark erode, but that would be engine out, fully stripped down and done that way.. But not sure that method is still used today..