Big
just a thought, you could use a friction drive looking channel. Mount the motor on top with a small gear box for needed gear reduction to a jackshaft using the bearings that are located for the friction roller.
Some friction drives use 1/2" roller shaft others use 5/8" shaft. Both bearings are 1 & 3/8" od. I have the part #`s. Most jackshafts are 5/8", easier to get different size sprockets. Belt drive may be easier at 1/2" shaft.
"bike motor parts", the new owner should be up and running soon. Their $49. Double jack shaft channel mount may have some options for you.
Run a belt to a whizzer sheave bolted onto your spokes. That will be the easiest with your coaster brake wheel. Belt is cleaner, quieter and may out last chain. Fan belts run for years on cars with spring tensioners.
Spring loaded belt tensioner is very good, and a lot eaiser on spokes and everything else. Full floating and self aligning = the way i like things.
Sprocket/chain and some pulley/belt systems have a tight loose charactoristic during each cycle of rotation. The spring loaded tensioners solve all that. As long as a builder does not use too strong of a spring.
Just my thoughts, not trying to be pushy. I know in the beginning it is hard to decide, too many options!!
What size is stamped on the tires, how wide are your rims?
If you want something very dependable use the double jack shaft motor mount for your gear reduction, no gear box needed then. Parts left out never break. I was going to order the parts for a double belt drive, but they are down until june was posted on the web site. I think you can type in "bike motor parts.com" and look at the parts on the website.
A housing can be bought that has a clutch drum, double shaft support bearings and bell housing that bolts on many popular motors and will accept a sprocket or pulley. The motor belt is run to a pulley on one side giving you some gear reduction there. The the belt on the other side gives more reductions.. The belt or chain going to the rear wheel can be under, in side the channel.
This is much simplier than it sounds. I have a neat 49cc kawasaki with the belt housing, will edit in a pic tomorrow. I have pics of bmp stuff if the web site is not up. I have pics of the pulley drive on my hard drive, but have posted them already, you can not post the same pics twice.
So many of the things sold are troublesome. As a senior, when i hammer something together i want it to last. If it is troublesome, i get my "bfh" out and cure it. Lol
ron