All NE cylinder have a larger intake valve [24 MM].
The camshaft MUST be advanced one tooth, otherwise the motor is way too rich.
The stock carburetor will work best if the camshaft is the WC-1 stock version.
The top end speed is controlled by the main jet, and most often the jet is too large, not too small.
Look at the sparkplug, if it is black, and you have been riding at WOT for a short period of time the jet is too large. If you ride at WOT for several miles, then check the plug and it is white, the jetting is too lean [small]. You are looking for a tan or brown color after running at WOT for several miles.
You should be able to reach 45 MPH if the motor is tuned correctly. The lifter clearance should also be checked........007" intake, and .009" exhaust on your setup.
Test the bike with the intake breather removed, as many of the early motors ended up with large amounts of oil in the breather element from the oil vent hose. If the breather is obstructed it will act as the choke is slightly closed [will kill top end quickly]. Also try removing the muffle insert and check the results, as the earlier insert is very restrictive, and mostly effects the top end RPM.
The correct 22 MM carburetor is very expensive [$130.00], however it is setup correctly for the Whizzer motor with the correct slide, needle, pilot, and main jet, whereas others are almost impossible to make work correctly. I doubt you will need the 22 MM carburetor unless you upgrade the camshaft & exhaust system.
Have fun,