Re: shaft drive
I wonder how strong the gear reduction in the shaft drive bicycles is. It holds up to pedaling forces but an engine can exert possibly 10x the torque on the worm drive or ring and pinion driving the rear axle. I've contemplated a shaft drive motorized bicycle but haven't pursued it due to the increased cost for diminishing returns, when I've had great service from simple chain drive setups using KMC nickle plated HD chain on the single speeds, and Whippermann Connex for the 9 speed shifter.
For those who don't like the messiness of a chain drive due to the grease, you can try dry wax chain lube with teflon, like White Lightning, which doesn't attract dirt.
Direct drive is definitely a great idea but I think maybe using a moped drivetrain would offer more trouble free riding without gear failures.
KTM50 Schwinn Chopper - 6000W E-Motard - 52cc Auger BMX - 9sp+CVT 52cc Cagllari Moped