Hello Fatts,
I also investigated about the mezzo electrification.
You're lucky because the rear dropout width is 135mm.. so you can decide between several motors.
What speed are you aiming at?
For geared motor, you can have the BPM, Ezee motor, the mac motor or even the cute100. I recommend you the cute100 and aim at 20mph max speed if you look at geared motor.
Because it's a small wheel, you'll need much higher RPM than the motors are made for. And if the speed is higher than 20mph, the gears may not be able to handle the speed.
I may try to get a geared motor at 25-30 mph but I'm not sure yet because it's very experimental and may kill the gears.
If you look for higher speed than 20mph safely, you have to go for direct drive motor. Except for the crystalyte G209 (around 4 kg), all motors weight at least 6kg and may be way too eavy for your purpose. (golden motor, crystalyte HS, etc...)
Also, you'll probably have gear but not 9 speed; I think you'll have the standard 7 speed associated with the motor to remain at 135mm
