We do not have a 36 Volt controller rated for less than 500 Watts however a 500 Watt controller can be used with a 250 Watt motor as long as the load on the motor is not too high. If the go kart will be used on relatively flat ground at low speeds then the 500 Watt controller should be okay to use with a 250 Watt motor because the motor will not draw more than 11 Amps from the controller.
We have a 36 Volt 500 Watt controller with reverse which is our item number SPD-36500R. For maximum speed control we have a Throttle Top Speed Adjustable Limiter which is our item number THR-ADJUSTER.
The SPD-36500R controller has a brake switch connector which turns off the motor when the brake is applied.
I checked and we can special order a disc brake rotor adapter that fits a 20mm OD axle with a 6mm keyway. I will have to check with the manufacturer and see what the rotor mounting bolt hole circle is on this adapter to see if any of our disc brake rotors will fit onto it though. I will send them an email and let you know when I hear back from them.
Please let us know if you have any questions.