Disclaimer- no horses will be hurt during this build.
I was poking through my " gonna use it someday" pile and darned if this idea didn't just jump out at me. The rear end is from a sears ss/16 and the bike frame is a Honda rebel. Front forks are from some Chinese scooter.
Here's the "kinda" plan. I'll need spacers to fit the rear forks to the frame, no big deal. I have a Briggs 5 horse horizontal from an old tiller that I plan to use. I'll flip the chain guard around and use it for a belt guard on the other side. I also have various bike parts such as clutch levers, etc.
This will take quite a while, since I'm short on time anyway. It'll be a few minutes at a time. So, without further ado... here's the mockup.
