I'm currently custom building a Moondog bike for my friends son. It's 26'' with a balanced 66cc bgf motor, cruiser bars and seat and moped tank and front brake hub. I like this frame for it's tight triangulation. Looks like the motor belongs there. I say custom building because not a lot of kit parts are used. I had to balance the crank, make the brake stays, cable length and clamps, tank mounting brackets, etc.
All my personal motorbikes are 4 strokes. 3 Whizzers, 3 cruisers with 5hp Briggs sidevalve engines on 26'' 70's Schwinn frames, most with moped forks, brake hubs and tanks. One has a homemade 2 speed automatic and variable timing, the other two have late Whizzer aftermarket autoclutches. Then there's my 8hp Briggs flathead also on a Shcwinn 26' frame and autoclutch. I also have a 24'' Schwinn vintage cruiser with a Stihl 4mix engine from a backpack leaf blower. It has a moped tank, rear brake hub and forks, motorcycle front brake hub and custom engine mounts with jack shaft. All of my bikes will do 40mph, some more. Most are belt drive with a couple of chain setups.
I have some 2 stroke, non Chinese motors I've been saving for bike projects in the near future. Stihl, Tanaka and Robin. I would also like to build a diesel bike.
I don't know exactly what you're looking for in this thread but here's some of my collection and maybe you can get some ideas. As you can see, I spend most of my time with DIY projects.