I have one bike with 300miles on it now that will hit a severly vibrating 39MPH with a 36T sprocket, it has a ported intake and an exhaust that I made, I built this engine from ground up after cooking it early on, (trashed the ConRod/Crank Bearing) I milled .012 off of the stock slant head and smoothed up the ports a bit and removed all the burs and such, better plug wire and NGK B6HS plug, rejetted the carb and all else is stock, this bike cruises very smooth with almost zero vibrations at 30-32MPH, very nice ride and would be much better on a better bike.
My other is a MT Bike that has the Straight Plug 6mm stud jug and it has the stock Alum. intake, a home built exhaust, all internals completely stock and only mod so far was just rejetting carb. it has 945miles on it now and has a 41T Spr., it will hit 34-35MPH but is not a comfortable ride past 27-28MPH with the 41T Spr. to many R's and the vibration is murder on the ole "duffle bag/marble sack" and anything else that is on the seat or bike in general, it is a very strong running engine and I plan to put a better intake and and raise the compression a bit by making a new head gasket out of some .016 Alum. I have, the stock gasket is .027 so by reducing the combustion chamber by .011 should up my torque a tad and then I plan to put a 36-38T Spr. on that bike as well.
Here is a link that will explain my upcoming pusher project, and this will get me in the 40-45MPH cruising speed I'm looking for, and yes I will be using a very high quality MT Bike with disk brakes and all the good stuff.... and No I will not be cruising at the speeds this guy is, that is just wicked fast on a bike and more than I desire...
Enjoy this one guys
YouTube - BICICLETA CON MOTOR 95 KM/H 60 MP/H PUSHER BIKE