It sounds like the fastener was turning fine - not rusted in or anything, and now that the load is off, it merely needs to be turned out. So, that's good news... anything that will allow you get a purchase on the remnant will have a very good chance of working perfectly. If there is ANY of the fastener sticking out, but not enough to get hold of with needle-nosed vice-grips, then welding a 'handle' on the nub, or grinding a slot across the face may well suffice to turn it out with a flat-blade. If it is broken off flush, or below the surface, you will have to centre-drill the fastener for an extractor, or with a reverse drill bit. (See Bofud's pics for an extractor) The tricky part with this is, of course, getting the hole you drill in the centre of, and in line with, the fastener. ANY TROUBLE and ALL OF THE TIME you invest in this part of the operation is well worth it, as the drill bit wandering into the threads and possibly off into your engine's sump or block is the nightmare scenario you want to avoid. If the fastener is an M6 or bigger, a ball-ended carbide grinder will cut a smooth depression into the broken fastener's top that will help you centre-punch the fastener just where you want, and be a good start to getting your hole centered.
If this is truly on the bottom of the engine, then some thought is going to be required, and some preparations taken, to get the access you need to put things right. Again, the time you take in preparations is always well spent. If the broken fastener is in a part that can be removed from the engine, then that may be justified, if otherwise access will be a real problem. Jeff would probably undertake this work for you, on or off the bike, if the whole thing seemed likely to go sideways on you...
