The trick that was used by the Urbaelectric from the 70's was pretty clever. They paired the electric motor with a commercial continously variable transmission and controlled the gear ratio from the accelerator pedal (they did that with a simple stepper motor but a mechanical cable linkage such as the one from the regular throttle grip would work too). As soon as the accelerator was depressed, a micro switch would be turned on and a set voltage was applied to the motor and it would start up at a constant speed determined by the voltage level, varying the gear ratio was what controlled the speed and reducing the accelerator caused the motor to brake regeneratively. Pretty simple and CV transmissions are common with many small industrial motors. The urba-electric also had three voltage settings to apply to the motor which had the effect of changing gears on a standard stick shift, this augmented the limited operating range of the CV transmission in terms of gear ratios, you could select those voltage settings with the gear shift pedal and the voltages would just be different arrangements of the battery connections.
A modern approach would be a chopper circuit whereby you pass the power supply through a mosfet power transistor (essentially a solid state switch) and flick it on and off at a relatively high frequency, the ratio of when it's on versus the on off cycle is the percentage of the maximum voltage that the output would be, a large capacitor on the output end (perhaps with a current limiting resistor to avoid disaster) averages out the voltage level before it hits the motor, that way you can vary the speed of the electric motor by varying the voltage applied to it. If you use an AC motor such as a very large alternator, you just make a solid state inverter perhaps salvaged from a commercial UPS and just control the frequency at which it operates. Unfortunately, most inverters are very inefficient using resistive snubber circuits to counter commutation but if you look around, you may find some with more advance snubbers. Plan on blowing up a few power transistors (mechanical relays aren't reliable enough and aren't fast enough). Unfortunately both the chopper and inverter approach won't handle regenerative braking and requires a separate circuit for that.
Potentiometers for speed control work by what's called a voltage divider which is a resistive device. It's fine for low voltage, low current but is wasteful for any usable power levels, however it can be used to provide a reference voltage level for use with an analog control circuit such as a darlington transistor/ zener diode set to drain the base to emitter voltage of a power FET when the FET output averaged out by a large capacitor exceeds the reference voltage which would be the determined by a voltage divider controlling the voltage level at the emitter of the zener diode. The current passing through the zener diode would be wired to control the base to emitter current of the darlington transistor. This would have an effect similar to the chopper circuit but the throttle would be a simple potentiometer and resistor instead of an adjustable pulse width frequency/impulse generator. I've used this approach as a simple voltage regulator for trickle charging batteries before.
A 5 to 10 horsepower motor should be plenty for a motorcycle, remember that electric motors develop torque immediately.