If that's not going to work, my other options are:
1) Keep the 24V 500W motor but go back to a 24V battery and controller and change the motor sprocket to increase top speed. Do you know where I can find bigger sprockets that will fit the motor shaft? What about SPR-2514A, SPR-2515A and SPR-2516 sold at electricscooterparts.com?
2) Use the original Razor 24V motor and controller but run them at 36V by adding a third 12v battery. I've done a quick test on my other buggy and that yields at least a 25% increase in top speed (although I was expecting 50%).
Thanks for your help.