The Alltrax AXE7245 motor controller is a good match for the ME1003 motor. It is proven, reliable, affordable, and easy to program. It operates up to 90VDC.
Alltrax also has a newer family of controllers called the SPM series which is functionally similar to the AXE series. The SPM controllers are larger, less expensive, use a non-sealed design with a cooling fan, and have an updated programming and wiring interface. I would have used the SPM72400 except that the smaller size of the AXE made it easier to fit on my motorcycle.
The motor controller delivers energy from the battery pack to the motor using pulse width modulation (PWM). The magnitude of the instantaneous voltage and current during each pulse is equal to the voltage of the pack and the current demanded by the load. But the high pulse repetition frequency (PRF) causes the motor to react in proportion to the average voltage and current of the pulsing system, spinning faster for higher duty cycles and slower for lower duty cycles.
The duty cycle varies from zero to one hundred percent, based on the input from the throttle potentiometer. The high PRF ensures smooth operation from zero throttle to full throttle. The controller can also be programmed with different output curves for best performance and feel.