Re: Introduction time.
If you mean the engine shown above, you're right. It probably looks best in a drop loop frame. But if you go back and look at some of the configurations used with this type of engine, it has been placed... in front of the down tube, inside the of the diamond leaned forward near the head tube, leaning forward near the bottom bracket, standing straight up, leaning back like the seat tube, and part of the seat tube. That was when the engines were samll enough to fit in those spots, about 1-3/4 hp. When they got bigger, say around 2 hp or larger, then they would sometimes only fit in a drop loop frame. My idea would be for the smaller variety. A kit that could be made to fit on either the down tube or seat tube locations as well as looking great in a drop loop frame. New technology would allow a 50cc size (hopefully) and a decent enough hp. I hope.
If you meant the engine that's currently in my bike, the 125cc Zundapp, then in order to fit in a drop loop frame it would need an awfully wide bottom bracket for the crankarms to clear the motor. It's pretty wide. That's why it sits so high in the frame.