I just use a junior hacksaw or a full size hacksaw to remove material, it'll cut WAY faster than a dremel, cutting aluminum with a cutoff wheel in a dremel doesn't work well because it catches and builds up on the wheel. before I pull out the saw, I simply use channel lock pliers to break off the bulk of things, or cut the end of where I want the material removal to stop and then use the pliers so I don't go too far.

you don't have to go nuts on material removal, cutting off little bits of aluminum doesn't remove all that much weight, so if there are little ribs that might help the structural integrity of the crank case, leave them... it's better to have an extra ounce of weight than to have a crankcase that will break in a few hours of use.