Re: 40 MPH Club
40 MPH, that's 65 K/hr,
I'm light weight, which helps,
But my Screaming Roo get's this easily straight out of the box.
A standard 38mm stroke Ebay motor takes a little work, but should get this speed with a few mods to the barrel and a standard carb, as long as you have port timing correct and compression up a bit.
My case induced motor with KTM pipe easily hit 55MPH, 10500 RPM on a 36t and 26" wheel, and it's not run in.
But to get that sort of power, you need a decent pipe, not a 'banana' pipe, or a modified China pipe.
My first motor, I got to 80K (50 MPH), with a home made pipe.
But unless your starting with a well balanced crank, preferably 40mm stroke, good luck!