Try to find a lower RPM (higher turn) motor than stock if you're going to run 24V -- it won't run as hot (nor as fast) , but at 24V you'd get performance similar to what you'd get if you ran the stock one at 18V. If you believe the specs on this ebay motor, it would be a good fit -- 22k RPM @ 24V -- unfortunately the specs are incomplete (like most of the aftermarket motors...)http://www.ebay.com/itm/DC-6V-24V-12V-High-Speed-Power-RS-550-Drill-Electric-Tools-Screwdriver-DC-Motor-/191930807571?hash=item2caff7b513:g:cwIAAOSw3YNXbfTx
I'm assuming the stock motor in the Rollplay cars is similar to the Power Wheels ones -- 12V = ~15-16k RPM, typically running a 28-33T "550" motor.
The best option if it work is to get a larger (775- or 755-series) motor. I don't know anything about the Rollplay gearboxes or mounting, so not sure it's possible.
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