What I suggest is that you open up the car, and take direct control of the motors with a motor driver (Could be the L298n).
Then, using WI-Fi or RF (Xbee) you can communicate with another arduino connected to the computer.
This way you don't have to deal with the single control board that most of the Rc cars have, and manage the motor control and communication yourself.
You can look for the model of the motors in the car and get the right specs.