Had alotta bikes, fastest was a 2 speed gripshift sachs with the lunatic airsal 80, it got over 60, highest I ever recorded was 63. it was very fast.
But apart from the very fast bikes I quickly came to recognize that 40 and reliable was much more where I wanted to ride than constantly pushing the limits, swallowing gas and destroying parts (clutches on that sachs) and sketchy late stops with way inadequate brakes. same bike later got converted back to 50cc and tuned up would max out over 45, which actually was pretty awesome, plus it got great gas mileage and worked without attention for ev er.
I'd say about maybe 3/10 bikes got kits, 2/10 stayed stock and 5/10 would get whatever cheap mods I could hack together, hackweld pipe, bigger or overbored stock or other folk's leftovers for a carb, and porting on the cylinder. That was usually almost all you needed to push at least to 40, and usually pretty reliably. Even the kitted bikes except a few where I was trying to really push it were all 40's range. steady and reliable, keep up with traffic and rally rides, carry 2 up no issue