Try some tips from this site:
ZBOX MOTOR FITTING INSTRUCTIONS
ZBOX TIPS AND FEEDBACK
And if you're worried about the rear sprocket not centered, then try the shifter kit from Sick Bike Parts found on the upper left corner. But you might find it even more confusing with more parts.
I used a nylon hose, slit it lengthwise, put it inside the hole of the sprocket to act as a spacer between sprocket and hub. It does not totally put it in center, but it does prevent the sprocket to wobble too often.