The one thing that's never mentioned- the derailleur on a multispeed bike acts like the chain tensioner otherwise- it adjusts the pedal chain and if there are no allignment probles otherwise- the motor chain tensioner can then be eliminated-
I recommend the Micargi mult-speed Pantera cruiser- the rear motor sprocket holes fit right on them with no altering, and a ridge stops the sprocket an eight of an inch from the spokes- there's no need even for the rubber then on that side. It's like they're made for the motor and sprocket otherwise- a trackkie flip/flop "fixie" hub is the same way, but may need to be spaced for a multi-speed freewheel, if a single freewheel isn't used. The track cog lockring is reversed threaded I believe and should hold the sprocket firm laterally at least- forward drive is then still needed- i'm hoping to drill a sprocket to fit directkly on a flip/flop rear hub.
A dual brake lever then is desirable on any bike with handbrakes, and works very well, and I personally like handbrakes, and don't want to alter sprockets for coaster hubs
Peadal assist may help on big hills, I like to pedal a bigger gear down from a faster motor driven speed, or shut the motor off to cool and save gas on long descents- not pressing fast all the time. I'm pushing close to 200 pounds, and my 36 is bogging down a little bit on the really steep hills here in Brown County IN, Hilly Hundred country- but still getting up them, and I expect to riding mostly on the flat soon,
and the 36 gives a good cruise speed-
a 40 tooth may be a compromise down from 44, and I've seen them at "Livefast motors" on ebay, in their upgrade kit-