It may help if you provide a bit more information about the vehicle. Do you know what brand it is? Do you know anything about the current motors in it -- are the 550-series or something different? Are there distinguishing markings on the motors that would help you identify the current capability? etc.
You said it was radio controlled -- is this a kid's ride-on toy or a smaller RC truck (in which case you'll be much better off in an RC forum).
You may also see if there is enough information in the Motor Database Sticky to help you or not.
F-150 - 24-36V with homebrew 100A variable-speed motor controller, 4x 775 motors (no, it's NOT all-wheel-drive--look HERE
for more details!)
Pink Rocket Princess Mustang - 15mph, 36V, 4x 775 motors, homebrew PWM controller rev 2....
Hurricane - 24V w/50A step-down controller, 775 motors