I built one of these three years ago. If I were to do it again, I would get the better quality motor from GruBee. Actually, no, I would find a 4-stroke.
I threw the cheap clutch lever away and got a nice one made for a dirt bike.
I got 115 mpg, 34 mph, and sprayed oil and raw gas all over the roads. My "sweet spot" was around 28 mph. I had to tighten all the bolts and studs every week, including retorquing the cylinder head at least once a month.
You don't need to modify the throttle to make it shut off. On mine, I just wound the idle mixture screw shut tight.
If I build another one, it will definitely have that shifter kit on it. I wouldn't build another one without it.
http://www.sickbikeparts.com/catalog/
With direct drive, the engine is screaming at 35 mph. Fuel economy would also go up quite a bit. That plastic wheel chain guide sucks big time.