Quite recently i got a broken Air Hogs "Havoc Heli" mini toy helicopter. it was brutally beaten with either a wall or a baseball bat, either way there was no styrofoam and the helicopter was lucky to have the electronics intact.
I was wondering what to do with it when I saw my large-ish collection of mini RC chassis'.
With no forethought I grabbed the most complete chassis and started orienting the parts. The battery went where the circuit board and Ni-Cd battery used to be and the new circuit board lay on top of the small plastic cover.
I took the motor from the rear propeller on the elicopter because it fit perfectly without any modification to tho RC chassis.
Then I soldered the wires for the tail motor to where the original motor used to go and resoldered a broken battery connection.
Then came the first test.

The result was a very fast little car, but it kept spinning out of control and the top wouldn't fit on because of the orientation of the circuit board and IR detector.
I flipped the board over and extended the wires on the IR detector.
Unfortunately while I was soldering the new wires on I melted the tiny power switch. And I had none to replace it with.
I found a spring loaded pressure switch and I added wires between it and the contacts from the original switch.
I glued the switch to the motor's pressure bracket (It holds the motor in) in such a way that the car would be on when the top was, this helps in the future because the crcuit board has no LED to signify that its on, so resoldering parts could be dangerous, now its safe no matter what.

Then came the matter of the spinning out. I tried several different sets of tires and it still spun out. Then I realized it wasnt the tractionthat was the problem but the front wheels. They werent anchored so they were free to turn whenever they wanted, at high speeds they turned randomly and the weight of the car kept them that way. The car doesn't actually have any steering.
I used my soldering iron to melt the plastic by the pivot pins for the wheels (I did my best not to breathe the fumes) and they were straight and true. It took a few tries because if the wheels' height was too different then the car would still spin out.
then I got some tires from my Zip Zap mini RC (it is sort of messed up) and put them on the rear wheels.
This improved the grip, thus improving the acceleration.

This is the end result, a extremely fast mini nitro RC car, i hope to get a video up soon of it in action.