I don't have the answer for you, because my testing materials for a similar project have not yet arrived, but maybe what I've found so far might help.
Depending on how the model itself works, it may not be controlled by voltage. What I've found is that servos, at least, are powered to a specific voltage based on their tolerance and desired output. The speed of rotation (or angle of rotation) is controlled not by voltage, but by pulsed information on their third (data) wire. So, altering the voltage supply would not have the desired result on the engine output.
The project I'm working on is a tracked vehicle controlled by a R/C car (steering wheel/trigger) remote. My plan is to take the trigger - speed - data, and apply it to the tracks proportionally based on the steering data. I plan to start by just running the receiver into the Arduino, and running the data from the inputs back out to the computer. Once I know what data the receiver is sending, I will use the Arduino to send duplicate data back to the actual servos. (I am doing it this way to allow me in the future to program automatic movement that can be overridden by the remote control)
I don't know if this is helpful, or even right, but hopefully it's a way of thinking that might be helpful.