clutch for small gear box??

I want to build a mid drive for a large cargo bicycle that is shift-able between 2 or 3 speeds for steep hill climbing. I do not want to use a V belt.

Most likely it will need a small motor cycle chain to a large sprocket on the drive wheel. Where can I get a 12 inch sprocket that I can bolt onto the spokes some how?

So I am hoping some one can tell me about some kind of small motorcycle gear box and how to set up a clutch. Or maybe there is a gear box that does not nee to use a clutch??

The bike will weigh up to 500lbs or more, fully loaded with cargo. Maybe 10mph on a 9% grade. V belt expansion transmission will not work for this project.

the more gears and sprockets there are, the less rear wheel horsepower there is.
Best to just gear it down since the heavy load requires all the power possible.
Put in better piston, increase engine compression, increase spark power with Jaguar CDI (which increases engine power), and make your own intake extension. also consider putting a good carburetor on it. possibly also a reed valve and Torque pipe.