You may want to make a gas RC electric as I show you for a few reasons. One methanol costs $30 per gallon after you drive to Hobbytown to get it. Two you don't like using fossil fuels for fun. Or three you have a gas RC with an engine that has low, compression like me. This can cost you about $40 on nitro rcx.com (not including shipping).
Step 1: Take the engine off
First you will have to take the engine, speed controlling servo, battery case, and fuel tank off. If you stripped the head of the screw too much I suggest using a cutting blade on a dremel and grind through the center so you can make it a flat head.
Step 2: Buy stuff you don't have
You will need a speed controller, a motor (try to get the cheapest motor with the highest kv rating), a battery (try to get the highest mah per price), at least 1/8 inch steel, and a gear that will screw onto your motor and still fit the gear pitch of the spur gear (the big gear).
Step 3: Build a motor mount
You will have to build a motor mount. First I measured the hight I wanted the motor off the chassis (it is better to have it closer to the chassis). You will either have to measure the distance from the gear to the motor mounts or make 2-3 holes about a 1/4 inch away from the gear on the side the motor will be mounted. I added the measurements plus 1/2 an inch for safety and to mount the motor so it won't go half way up the motor. Drill the holes at the right spot to attach the motor.
Step 4: Add tension
I mounted the motor correctly , but I learned the motor had so much torque it could bend the body enough to strip the gear. I cut a long piece of thin gauge metal so it would make the motor and the gears at the right tension.