I built a gas powered inverted push scooter using a chainsaw for a motor, two customized pulleys, and v-belt. I later added patios instead of the foot pegs and an aluminum cover for the rear end. It worked great and my son would go about thiry miles an hour, and me being almost three times his weight went about 10-15 mph. I made a few changes to make things work and will try to list all the changes as it has been a couple years past, and I no longer have it.
Step 1: Later mods added for safety and comfort
I later added patios to stand on because the foot pegs were a bit uncomfortable after awhile. Then I added the fairing from aluminum sheet and painted it red to match the scooter. The fairing helps protect the rider from getting anything like feet, or, shoestrings caught up into the rear wheel assembly.