So, the motor mount bolt is broken (one of the three) where it screws into the block?
Is it on the driver's side (under the exhaust manifold)?
Is it broken flush to the block or is a little nub sticking out (possibly within the motor mount hole)?
First, check the other two bolts on the motor mount. If they are loose you need to remove them one at a time, 'chase' the threads with a tap & die, and make sure they can be re-inserted easily. Do this one at a time.
Then, prepare to remove the other (broken) bolt.
Best way is to support the motor (jack under the oil pan with a thick board to spread the weight). You may then loosen the other two bolts, and the bottom vertical mount bolt (which cannot be removed with the exhaust manifold in place). THEN you may be able to move the motor mount pedestal away far enough to grab a "nub". Attack the nub with a vise grip, mine is a thin-jaw type that can get into a 1/2" wide space.
If that fails to move the broken stud, you will need to try an extractor. Drill a hole in the broken bolt (the motor mount can act as a guide so it won't slip if the other two bolts are holding it tight). If you use a reverse ("Left Hand" drill bit and are lucky, it might come out.
13 Piece Left-Hand Drill Bit Set
If not you will use the hole you drilled on a regular extractor.
I'm not sure about drill access, you might need a right angle drill.