Im a little confused by the information here. A hall-effect sensor could measure the speed of the wheel, or it could measure current to the motor. It does not regulate power to the motor. Hall sensor are very easily added to gears, so they would be suitable for a bicycle (in fact, if you look under most car youll find something that looks like a gear attached to each wheel; this i there for the benefit of the hall sensor that activate the ABS when a wheel lock up). The sensor you are looking at probably outputs a 4V square wave that varies in frequency with rotation speed.
Another answer mentioned a PWM controller. This is probably what you want. Its just an ordinary "motor speed controller," or ESC, which you can buy online or at a hobby store. You need to match it to the voltage and current your motor will draw. Remember that current is proportional to torque, so you will want to set up some kind of overcurrent protection for situations where the motor is overloaded; otherwise youll fry your ESC.
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