Assumptions at this point:

1. The platform and body have been completed and tested....

The drivetrain could be described functionally as very similar to an e-bike. The motor assists or replaces the pedalling effort when needed and  drives the same axle as the pedals. This axle in turn drives the outboard 3-cog sprocket which pulls a long bicycle chain through the 3-cog hub on the left rear wheel. Derailleur gear changers are provided front and back for a total of 9 speeds. Pedalling and motor drive are independent of each other and coasting is identical to a bicycle due to use of 3 freewheels:

• The normal freewheel that is part of the rear hub on any bicycle. Note, however, that the original bicycle rear hub is modified substantially in order to provide only 3, but larger gears on this hub. Why do that? Because the weight of this vehicle is multiples of the weight of a bicycle and requires a larger diameter set of rear hub gears in order to provide the extra torque to compensate for this added weight. 

• 1 for pedals in order to allow independence (coasting) from a motor-turned axle.

• 1 for motor in order to allow independence from a pedal-turned axle.

The motor system is the typical BLDC (BrushLess Direct Current) combination of batteries, controller,  motor and throttle (combination on-off key + hand twist grip).

For a visual summary of this drive train, refer to photos below:

1. from left side (vehicle front end was propped up in this pic):

2. ...from underside of chassis:

3. ...and from under rear of chassis,  the modified rear hub with extended derailleur mount (explained below):

Due to the importance of this system, all of these parts and assemblies are detailed in the OnShape model and drawings here.

The main (motor) electrical system design is specified here and is a copy of the motor manufacturer's instructions. Use it as a guide! Your components may be different so adapt, don't adopt!  This base system provides a 48V 2000W motor. 

For 2 reasons (see below) another 12V lead acid car battery is added  to this motor circuit and is used in a "momentary boost" mode via a handlebar-mounted  switch.