Can you say Riquimbili !

I think that's a very clever and simple set up. Most of the effort would be in finding the right bicycle to start with. I'd hunt for one with the dismountable
pedal levers that mount to the short crank using those tapered pins with a flat side to them. If the left pedal lever could be cut so to form a mount the pully could be welded/bolted to......then the pully assembly could be reattached with that pin and secured with a nylon retainer nut. It may take a socket with extension/swivel to secure the nut but I'm sure it could be easily done.

The welding of a plate support for the motor between the front diagonal frame tube and the seat post would not be difficult at all, not to mention the foot pegs. (but get a skilled welder for this who knows his trade and explain what you're doing so he can reinforce the engine platform)

Selecting a motor with a centrifigual clutch and adjustable carb would be the next chore. With fall coming alot of yard equipment will be showing up in flea markets at the end of season. Often those who mow commercially sell their
stuff off at the end of the season so they can buy new at the beginniing of the next season. You may find a piece of machinery that's worn out/damaged with a still usable motor for a project like this. (if the project works out to your satisfaction.....you can always overhaul the engine during the winter)

This would be an interesting project with a 5 speed rear shift especially riding around hills. If gear #3 was your default starting out gear you'd have two over drives and the ability to really run the flat stretches of highway.

The only thing I would do to the bike in that picture is BUILD A PULLY GUARD
and enclose that primary belt drive. It wouldn't be at all hard to do. A piece under the motor mounting that sticks out just past the pully's would have a flat piece of metal with several holes welded to that. Then a pear shaped piece of sheet metal could be cut the belt profile and around that some aluminum angle 1"x1" angle could be attached to it for a lip. The guard would attach at the center between the pully's with bolts etc to support it making the thing safer to ride if you were wearing pants. But all of the primary drive needs to be covered as one of those two pullies will be turning in a direction that would pinch or pull you into it.

I could see an emerging market for these things with younger guys who want something that nobody else has. (and they can proudly proclaim...."I Built It")
They may have to license and insure it as a motorcycle but it would be a fuel efficient bike where parts wouldn't be ultra expensive or unavailable. It's no wonder that when I was a kid I'd see little old retired men on motorized bikes.

Thanks for posting that Starrunner......I've been considering a Riquimbili project for my next bike and I have just the bike on hand that would lend itself to doing this. Just need to find a motor with clutch and some control cables etc. I like everything about that. The primary belt cover would really set it off. The seat looks like it's off a stationary Gym bike too.....extra comfy.