I have one of the first edition Ambassadors and 50 MPH is possible. Not sure the later versions with the CVT can reach near 50 MPH, as the CVT "eats" a lot of the power.
It also make a difference which output pulley you are using.
The later editions had 16 MM carburetors, whereas the earlier versions had the 22 MM units [mine is using an early 26 MM NE5 carburetor].
Check the cam timing also, as for some unknown reason they started advancing them two teeth instead of one????????????
It is important to note the speedometer is way off with the 3." tires instead of the 2.125" the speedometer is setup for.
My kickstand also broke and required a larger bolt to fix it correctly.
Have fun,