Hi Poshadetree
If I understand you rite you are talking about a toy remote control toyota, if so you could cut the feed to the motor and fit a voltage regulator in line with it at a voltge lower than the battery e.g. if the battery is 8 Volts fit a 4 Volt voltage regulator and it will go at half speed (but the power will also have half the power to go up slopes). Hope this helps.
Shane