I had an old rc helicopter lying around in the house which had lost its balance sometime ago. It would wobble badly and then smash into a wall and then drop to the floor. The thing was really cheap and low quality. However, the propellers, the motors and all electronics were good and worked. If you have anything like this that is ready to be thrown into trash, try doing this. It's real fun and quick.
Step 1: Scavenge the Internals From an Old Helicopter
Take out the electronics. The chip, motors and battery, everything is very small and light. Save them for later. Make sure everything works good. You may have to solder wires since they break from the chip easily.
Step 2: Body and Wheels
This is where you'd be making the platform of the car. Use styrofoam from old packaging boxes. Find something rectangular. You can use any type of wheels. I used some old ones that I had, don't even remember where they came from but sure were on some toy car. Same goes for axles, use steel rods that would be installed already on the wheels.
You will have to make mounts for the wheels. I used some metal sheet that was once a sweet box. These are very thin (just like the ones chocolates and cookies come in) and can be cut with scissors. Make sure not to cut your hand or fingers while doing this. The pictures are attached. I used a 12V drill to make holes in the metal sheet so that the axles can pass. Use double sided tape to attach the wheel mounts to the styrofoam body.
Step 3: Frame for Propellers
You can have propellers at the front or the back. I put them on back and they look cool. To make a frame, use old chopsticks. They can be joined with superglue pretty strong. They will support the propellers, the motors and the propeller axles. I used my same drill to make holes for these axles too so they can hold the axles. You'll need at least 2 chopsticks with holes to support the axles so they move freely. Make sure to align the holes properly. Attach this to styrofoam body with either tape, or metal clamps, like the one in pictures. You'll have to make them pass through the styrofoam and make a loop. Don't use superglue on styrofoam, it dissolves it.
Mount the motors on the wooden frame and align everything so gear tooth fit in perfectly. You can use superglue or depoxi to mount the motors. I left the plastic cover for the motors on and attached that rather than the motor itself so I can take out the motors if I need to.
Step 4: Add Propellers
Now mount the propellers on the axles. In this helicopter, the two axles ran concentric and one was hollow, the other fit inside that. I kept it as is. The only problem is that the non-hollow axle does not fit directly into the prop, so you will have to superglue that. It may take a few times before you get the balance right since it spins at high speeds.
Step 5: Final Touches
Now put in the chip, battery, cover it with tape or paint and have fun. I added the helicopter body to the front to make it look cool. Also note that I had to add another bit of styrofoam at the back to raise up the propellers since they touched the ground without it. It goes really fast and the differential motor control, which in the helicopter controlls the yaw, now controls the turning, though it is not effective at lower speeds.