Adding the Spring
I was able to ride the bike at this point, but I didn’t get very far: after a few miles of high speed bumps and vibrations, the motor brackets loosened up. After a bit of time thinking up solutions, I ended up buying a spring from a local hard ware store. I rigged up a wire hook to a hole on the bikes fork and a brass hook on the far end of the plywood plate between which the spring can be tensioned. The spring allows for changes and movement of the engine while keeping plenty of traction between wheel and motor.
All of the parts for the handlebars were scavenged. The gas lever was originally a brake pedal on a free bike I had found during one of my yard-sale searches. The irony of using a brake for the throttle should not go unnoted. In fact, it's a borderline bad idea considering this bike has coaster brakes and I'm used to quickly squeezing brake levers for panic stopping. That could be a nasty surprise… The brake levers originally slipped over the ends of the handle bars. Since I couldn’t slide the lever into its new position, I instead cut a section of the levers clamping attachment. It still allows for a tight fit without having to remove the handle bar grips.
I was able to use the original cable form the brakes of the donor bike, which was plenty long. The end of the cable was held down in the same way as the original throttle cable. To attach to the throttle lever, I crimped on an electrical terminal with an eyelet and used a small 4-40 screw to bolt the terminal to the lever.
The kill switch is very handy. For example, you can let the engine to cool down when it starts overheating and pinging. It can also let the engine assist with braking which is a helpful safety feature. I used a simple momentary, normally closed switch with a flush mount housing and basic copper wire for this task. The wiring just connects to the two terminals on the coil to allow the switch to interrupt the spark when pressed. The switch is mounted into a piece of plywood and fixed to the handle bars with an aluminum bracket and a single bolt.