I made a small bit of progress on the Ninja. I verified from the manual that the key was a Woodruff key, which is more or less a half circle. Got a punch set from Sears. Knocked the back end of it down a bit with a center punch, then finished up from the front with a chisel. Autozone sells the keys (the HELP! series) in a set of 5 for about $4. You can get them for about a quarter if you buy them in bulk. Anyway, it looks like the one I need is 1/8" wide by 9/16" long. Mosquitoes are out in force from all the rain, so I decided to hold off a bit.
Next step is to figure out how to get the new key in without damaging the crank any further: There are some scratches already from the first key shearing off. If I'm careful to protect the surface with rags, I might be able to use pliers; I think that will be safer than hammering (less stress on the bearings). The end of the crank is not all that big.