Bobby, one issue you are going to have is getting a motordrive that aligns properly with the baseplate of the Spotmatic. If you can get an original Spotmatic MD then you are probably OK, but anything else is going to be a real problem. The distance between the tripod mount hole and the drive connection has to be the same for it all to connect properly.

FWIW, Pacific Rim Camera (http://www.pacificrimcamera.com) have an ESII drive listed in their catalog at $195. For another $250 you could get a 250 shot back too. You would still need the battery pack though. I would expect all of the Spotmatic era MD units to be the same size and probably the KX MD and K2DMD units as well, but all of these are collectibles so will be expensive.