I know your post is a few weeks old but I just joined the forum today.
I want to add an additional answer to the ESP Support reply. I agree with all he said but I just went through trying to further regulate the speed of the 500W motor on the kart I just built for my son.
You can wire in a potentiometer on the throttle signal wire. I've recently done this. Works really well. I've read on some other forums that you can't do that, but I can tell you it definitely works. Mine is a hall effect foot pedal. Just a simple little variable resistance piece you can get at radio shack for $2. Get the 10ohm. You turn the dial one way and it limits the power coming down the line = slowing or speeding up the kart.
The other option is getting a controller that has one built in. I got one of those from our host here. We won't discuss why I have had to buy multiple controllers though. :-) I made a little mistake but not related to the potentiometer though.
A 24 Volt 500 Watt controller can handle a 500 Watt load continuously however will output a 720 Watt load intermittently for extra power to get up short inclines and accelerate from a dead stop. As long as the two motors connected to a 500 Watt controller were geared and used in a manner where they do not exceed 500 Watts of continuous power draw then the 500 Watt controller would not overheat and be reliable.
Keep in mind that when overvolting a 16.8 Volt motor to 24 Volts its RPM will increase by around 40% so the gear ratio may need to lowered to keep the motor running at a high RPM so it does not overheat itself, or draw too much current from the controller and overheat it.
Update 4/18/2018: We now have 18 Volt controllers available at this link: https://www.electricscooterparts.com/speedcontrollers18volt.html
With our controllers for brushed motors, when a brake switch is connected to the controller and used the controller will shut off the motors, however, it will not apply any braking force to the motor.
Since ESP brushed motor controllers don't actually brake, could I short out the motors via relay when the ESC brake is activated? I think that is what the motor braking of Power Wheels does, but I'm not sure.