I've been researching this idea closely too, been examining the dimensions of my schwinn Stingray as my pet project for the winter, want to get the largest possible 4 stroke I can in it without cutting the frame or welding it, the only thing that may get toasted is the pedals, they might become just foot rests with a lever built into one side for the rear brake.
So far the doodlebug db30 motor (a Honda gx100 clone) is looking like my best candidate. I can get a manual clutch (Yea I like em) that will be affixed to the output shaft, then some fancy jackshaft rigging and I'll have a nuvinci hub most likely in the back, or possibly an internal geared hub, have to say the nuvinci has been holding up to the relative hell I put it through, a good winter, being stored in freezing conditions (I had to break the bike free of an armor of ice, and 3 quarters buried in snow. Then used in that cold state without prewarming 3 miles to work since the roads were gridlocked with incapacitated vehicles of all sorts, including a jack knifed tractor-trailer) broke a couple spokes with it too, rain, sleet, ice, salt and grit, going from freezing Temps nearly directly into hot operating Temps, a very powerful 2 stroke pulling my 210lb hindquarters up some ridiculous Hills.
Anyway, the hub in combo with a clutch of some kind with a 4 stroke should be a great way to get around. The doodlebug motor is a 97cc, which is much bigger than the very common 49ers, has more oomph than a 79 cc predator, but still small enough for my task, while also having enough punch to feel more like a 66 2 stroke, but with a better power band, and of course the many many advantages of a 4 stroke.
It's just I never really dabbled with 4 stokes much, so I'm intimidated by the idea that if something goes wrong I will have no real idea what to do, and I don't know anyone except a guy in the next town over who knows anything about 4 strokes (he buys, repairs, and sells riding mowers)
Also going to be forced to use a different chain type for the clutch, it's a #35 sprocket, so I'll need yet another chain breaker, then I'll use a #35 sprocket on a 5/8th keyshaft, then the other side will be a normal 1/4th pitch chain, as thick as I can get for the thickest sprocket I can get for the hub, this time I want to go with some serious durability.