Actually, since the original MM pickup was essentially a MusicMaster, Duo-Sonic or Mustang guitar pickup (same pickup: a Strat pickup with a little less wire and flat magnets to go under a solid cover), any dual-rail medium output Strat pickup will work well. I say dual rail in order to get a slightly curved rail rather than flat polepieces on the original in order to match volume string-to-string over the crown radius, and to be humbucking, as we all know a standard Strat or Strat-style pickup is as noisy as anything, especially when playing where grounding may be an issue and sources of electrostatic noise, like neon bar signs, abound.
The other reason: does the OP's MM have the original factory wiring? The original circuit on a MM bass has an extra guitar tone capacitor bridging the circuit, so even with the tone dimed it is essentially at "8/10." So if the bass has the original factory circuit, then a brighter pickup to keep definition is in order. If it does not have the original circuit, and only has a "conventional" V-T setup with the standard .047 tone capacitor, then just about anything will work.