I worked on my bare frame/mid-engine buildup today. Scrounging used parts laying around. 4.5hp CY460 engine. Spare crank arms, freewheels, sprocketS. 5:1 tranny. Fiddling around with mid-engine placement. My Scooterguy-mounted bike sidles next to me for comparison notes and measurements. It is an excellent commuter bike. I have to fight the urge to clone it.
I need to configure it in such a way that the SBP/Happy Time expansion pipe will fit tightly. I have the stock muffler (loud), a Dominator pipe (super-loud) and the SBP/Happy Time pipe to choose from. The SBP pipe is the best one to fit into the build. Even tho the engine port and exhaust flange is grossly mismatched (rectangular engine port/ round pipe flange port), the pipe works extremely well. Besides quietening the loud engine to the point where you can hear the engine's reciprocating parts, it makes excellent power.
Back to fiddling.