May save a bundle of money getting a set hardened over getting custom gears made.
The DLC coating is to act as a wet lube bath replacement on the hardened gear. It has some pretty amazing anti-wear properities, both on it's own surface, and the surfaces it meshes with.
Maybe a hardened tooth profile, micro-polished finish, then DLC coated. Here is a white paper about DLC as a replacement for lube on high pressure hardened surfaces here:
http://www.thefreelibrary.com/Scuffing+resistance+of+DLC-coated+gears+lubricated+with+ecological...-a0216178973
I would bet under $150 for the hardening, micro-polish, and DLC coating all combined. Maybe under $80 if you can find a single shop to do it all in-house.
For the sloppy keyway problem, perhaps assembly with a little metalized liquid epoxy? Something like the devcon for steel? It can do wonders for getting some more life out of worn splined connections, never seen it used with a keyway, but the multi-sized metal particles in it lets it tend to stay in place under compression and pack-in rather than walking out like normal expoxies. On a fresh motor with a fresh tight fitting keyway, perhaps just some de-greased surfaces and a bit of loc-tite could be enough to stop the wiggle, which stops the wear. This type of situation doesn't seem like the keyway is inadquate for holding the torque, but rather just that the play/wiggle lets it wear. Loc-tite may be a $0.02 solution.
For disassembly, just heat-gun the loc-tited area to about 250f and it softens up and slides apart.