Quick answer: get the same one. I'll bet the $20 or so you pay for it would be far cheaper than the time it would take you to modify the circuitry and firmware.
Long answer:
You'd want the same wattage, torque ratings, and step size, I'd imagine.
A wrong wattage would be hard to compensate for.
Most other things could be finagled if you dare. Wrong torque or speed could be compensated for with a gearbox, or even with firmware. The firmware just sends 3 signals to the stepper for every step. If you modified the code to send those signals slower or faster, I would think you could effectively modify the rpm of the motor, and thus the torque.
Keep in mind though, if the second motor is different, you'll have to wire it in separately, and probably add some resistors to balance out the current going to each, plus write a separate code for the second one.
