For the tilt sensor I would have thought you could just use a lightly-damped pendulum with a limit switch. Alternatively, a design I've seen used for impact sensors is a cone with a steel ball in it, that closes two contacts as long as stays at the bottom of the cone. Once the G exceeds some threshold, the ball rolls and breaks the circuit.
I'm not familiar with that speed controller. Does it simulate inertia? If not, that's something that would be feasible for the Arduino to do based on a wheel speed sensor, so that you get scale realistic acceleration and deceleration.
For the amount of effort involved in making something like this, I hope you'll end up with a realistic engine sound emulator too. 