For years now, I have been considering a mid-mounted freewheel crank drive.
Running a geared reduction motor through the 7 speed freewheel sprocket, or freehub cassette.
Most any bike could be converted, adding less than 10lbs of motor hardware. (Battery additional.)
Typically, this involves using a geared reduction motor connected through the front chainring.
A big improvement over most any type eBike drive!
With a 11-34T 7spd freewheel you can have the equivalent of a 7 speed motorcycle, with +300% the torque at low speed.
Great low speed torque and high speed assist ... right to 30mph(?) ... from the same motor!
An important option is a 2 chainring freewheel crank.
This allows motor power without pedals being turned ... much safer!
There is the variable of matching motor to pedal speed.
The size of the 2 chainrings can be varied, and / or voltage adjusted, to match your rhythm.
The next stage option is a freewheel sprocket, instead of a fixed sprocket on the motor, this allows easy pedaling when motor not engaged.
My project motor is the Unite MY1018z gear reduction motor.
I've poked-prodded-tested and tried this model for ~20,000 miles.
As a demonstration,
I graphed the speed and torque available from running the motor through the 7 speed sprockets.
MY1018z motor @24V w/9T sprocket
At 24V Low gear (34T) will give you great power-acceleration till 7mph, great for the worst hills.
Then, there are a variety of optimal hill climbing, or acceleration, choices till about 16mph.
Of note is that a 34-13T would limit maximum speed to the "legal" 20mph.
I just finished purchasing the individual components and will continue to post build reports.