If its a rat bike anything goes. Till I knew I could buy a sort of motorcycle style tank I put a lawn mower gas tank mounted on the upper frame. The looks ehh, but one thing is plastic tank as it is won't rust. That's a plus!
Also, when I get the time and place to finish building the art fish cover, it will get a coat of paint after surface rust is sanded off. The under 5mph with a high ratio and a fire proofed cloth cover will enable it to look like a California Sheepshead fish for parade events.
I was wondering the rear wheel sheave, is it a pulley like a Whizzer or a clone of the Whizzer new company make. I used to use a washing machine pulley back 40 years ago on a bike that was pressed steel. Recently not available unless you pay custom manufacturer to make, I ended up using aluminum washing machine pulley and it split it after I raised my ratio to get better torque for trail and hill climbing.
I bought a Whizzer clone steel pulley and have it working OK on my rear wheel now. It stuck out further and my knobby tires then required I mod the rear stays which worked out OK too.
The pulley on your rear wheel if it is as maybe I saw in another thread link to a picture seems that it is maybe a smaller rim mounted to the wheel your using. If so maybe it is slipping there. I do not know. If the one on the jackshaft is for A section and does not slip, but it is the rear pulley slipping, then maybe the belt edges are not grabbing.
If I am right, not about what's slipping or not, but that the A section belt is not supposed to grab by the bottom of the belt in the pulley, but on the walls of the pulley on up to the top edge.
Others on this site have used rims to get large pulley effectively attached to the rear wheel, but I have not done it myself. If this is the case that you are using a smaller rim for a pulley, check how others dealt with the issue of slipping.
If you get a chain and use on of those marketed ragjoint mounts for a rear wheel chain drive, maybe there are other issues to deal with.
I being one that liked to make as much from scratch found that my washing machine pulley part could not be replaced with a steel part anymore a resonable cost, so I went with a Whizzer clone part.
MT