There are a handful of things that tell me it is home built. At ir near the top of that list is the recoil orientation as Fleetwin said. Ithe also seems unlikely they would have used their standard at that time oil bath air cleaner for horizontally oriented vertical shaft engines on one that was intended for an outboard that should be able to be tilted. I’d imagine this could be tilted to the point of draining much of the oil out of the air cleaner. Besides that, most outboard motors do not even use an air filter at all, only an air box to quiet the intake noise or just a bare carburetor. There are more but those are the two biggies.

As a guy that spent his youth building mechanical contraptions of all sorts, I do like this creation. It is very clean and well done. The more I look at it the more I want to build one myself. 😀
-Ben

That one looks factory, Tubs. Oddly, the recoil orientation is still perpendicular to the boat axis. ❓
That one does have the foam air filter though. Dig that crazy jet drive too. Looks as if the silver color is correct, but then the rototiller engine was the same silver and the rest of the OP’s outboard is natural metal colors of aluminum and copper.

I guess prototype is a possibility. As far as the workmanship, it is very clean. I would have to imagine this was built by a machinist / toolmaker and not by the average guy in his garage with a vise and a file. With decent equipment that level of workmanship is completely attainable with a bit of talent and pride in one’s work.
-Ben