This is my 1st Proto for elimination of one of the rear drive chains. 3 chain design. This is not easy from scratch in your garage. At least not for me....
Custom tank, fill in the void between the frame. You could use the frame from my bike as guide/template. mine is a little ghetto, but with the right tools and some patience, it could look really good. I will try to find a picture.
Yes... One speed. Direct drive is more efficient. Lose some hp by 3 chain design. I still goes 27 mph on a flat, Sea Levels love this.. NO DRAG from pedaling alone without motor. & Hey Large F. I tried to do a cluster shifter..&%($##$$) with this setup you really need just one Chain path. I tried 7, 4, & then just 2 Even the narrowest most flexible chain had troubles with any motor driven NON Parallel Chain Path. I have exhausted my ideas to do this especially on this bike. I like the way you think though.
The advantage is a LOT less drag when you pedal it with no motor. It also doesn't need a chain tensioner. Strectched frames really end up with long chains that are problematic over 20 mph. especially when the drivechain has to run over a roller for a direction change to clear a frame. This setup has shorter than normal length chains that run straight & smooth. Even though there are three the total length of all 3 are 25% shorter than 2 chains.