One of my fastest bikes is running a NT Speed Carb as Tom suggested above, it has good power and I can cruise all day @ 43+MPH on the flat without a head wind, I weigh 215lb currently.
I'm running this carb on a stock Dax GenIV lower and I have done all my own rookie port work on the jug and I have a Cat eye pocket bike expansion pipe with about a 12" header pipe going to it, a stock CDI ignition and a 34T rear sprocket on a 26" wheel, I had a 70cc hi hi Puch head on this engine before putting a 6cc fred head on that I bought used from a forum member last year, the "fred head" made no realized performance gain over the Puch head but it's shiny and will be replaced by a Jakes Diamond Head when they are available...LOL
I said all of this to let you know that with the right engine and set up, a reliable bike that will cruise at 40+ mph can be had with a simple and inexpensive NT or NT Speed Carb, no big honking carb is required to have a 40+ mph bike if everything else is right, and on the other hand if your bike wont do 40+ mph with the NT carb, it wont do it with a bigger carb either because most of the time the carb isn't the limiting factor for good power and MPH with these China Girl engines unless the engine is extremely modified and very well balanced.
A good-well balanced 66cc with a well tuned carb, good exhaust that isnt to restrictive, a correctly ported jug, enough compression, wheels on bike that roll easy, correct gearing and the right fuel/oil mix = good performing china girl bike build.
I can tell you that no matter what you do you will never get the power, torque or speed from the 49cc as you can get from the 66cc, but you can get all it can do from a NT Speed carb that is tuned right.
Old Hot Rodders saying is " There is no replacement for Displacement" and this also applies to these little china girl engines when it comes to power, torque and top speed.