Off the cuff idea, here. Years ago I wanted to build a belly tank car, but it didn't happen....however.... during my planning I realized that a solid rear axle (with the wheels wide enough for a belly tank) would probably eat up some of the power from a very small engine.
I think a person could get around it by using the rear axle from a small rear wheel drive car, with the pinion removed (and hole plugged), flipped over, and the back of the axle housing cut out for chain runs. This would allow for a differential, and you could run sealed bearings in the carrier to eliminate oil in the housing. Removing the ring gear would leave room for a sprocket.
The rear end I was looking at was from a Starlet or early Corolla 1200, because the ring gear size was only about 6" on those (so I could get a small enough sprocket to fit). You could also use old MG Midget, early Datsun 1200, etc. I figured it'd make it easer to mount an axle, because the old RWD housings are fully welded and plenty stiff. Brakes are already there, also, so that's one less thing to figure out or buy.
Just an idea....
Good luck with your project!!
JimL