A few weeks ago I saw a props guy at robins hobby trying to do the same exact thing... Was that you?
Have you considered maybe gutting it and connecting an arduino and motor driver in its place with a futaba receiver? I've done it using the wild thumper board from spark fun. It has an hbridge driver to run your tanks two dc motors and an arduino to receive the receivers signals and run the motors plus all of the standard arduino outputs for the other switched functions