Here is a link to a modified toy car forum where you may be able to find an answer to the motor/gearbox question: http://forum.modifiedpowerwheels.com
Upgrading from 12 Volts to 24 Volts would double the power and top speed of the toy car however in doing so it would also double the amount of heat that the motors need to dissipate which may possibly cause them overheat and burn out.
If you like to modify things for speed and don't mind replacing parts if they fail then this would be a good experiment to try. If you want to play it a little more safe though then the 12 Volt battery could be replaced with three 6 Volt batteries to increase the Voltage to 18 Volts instead of 24.
Please let us know if you have any questions.
i have one of these....http://www.kidscaronline.com.au/index.php/range-rover-evoque-12v-ride-on.html - not sure why this link isnt working when i add it in here. its the single seat version
i would like to get more speed out of it. What motor/gearbox do you recommend?
Also, what are your thoughts in upgrading from 12v to 24v by adding another 12v battery and running it in series?