Drill and tap a hole right in the middle of it, then run it out with a screw. Conversely, you could drill and tap it shallow and use the screw to pull it out.
Devastator's trick will work, you may have to file the first few threads off of the screw since it'll be hard to thread the key all the way through.
If the key is too hard to drill/tap, use a small cutoff wheel on a Dremel to slice it lengthwise (of course you should try to avoid overcutting into the crank, but it's not a big deal if you nick it). Use a large pair of diagonal cutters to squeeze the key together and lever it out like you would a cotter pin.
I always use a puller plate, but that's more to keep from damaging the bearing. I can see how the teeth might need attention with a jeweler's file after yanking the gear off with a 2- or 3-jaw puller, but if it got "destroyed" it was probably too tight a fit to want to reuse anyway.
