What I'm currently doing is a left side drive for the motor. The wheel is very narrow maybe 120mm or smaller. Right now the cheapest option I have is a spoke mounted gear. I can get the hardware at the link below. I've got to figure out what chain there using in that pic. I can probly shop around for something cheaper.
http://www.kingsmotorbikes.com/bicycle_parts.htm
The goal is to buy the pack and parts for less than $100.
So currently the plan is left side drive chain running to an intermediate shaft that has a coaster built in. Intermediate shaft is driven by the motor.
Current gear reduction calcs as follows
2500rpm motor 11t gear
27" wheel with 44t spoke gear
2500rpm motor w/11t to 44t = 625rpm @ Ishaft. 44t & 18t coupled at Ishaft. Ishaft 18t to 44t @ wheel gives 249 rpm x 27" wheel hieght for 20mph on power alone. Theoretical of course. Is that complicated enough. Sorry I don't have visual.
8mm 44 tooth gear $20
spoke gear adaptor $12
44 tooth spoke gear $13 + $5
total $50.
Currently spoke drive and gears are going to cost me $50. Unless I find them cheaper. I have a real soft set of EXIT light batteries that I can use for a temp pack. I know I'll need to buy some chain at some point.