Any 50cc unrestricted will hit 40mph, even chinese duds. If they didnt then no-one would buy them world wide. Honda 50's etc were capable of 40mph fifty years ago. If you check out the horse power of the machine you will see that 40 is realistic. My a50 hits a gps checked 41mph on level ground with no wind and I weigh 254 pounds.
The variator has an effect on top speed due to its profile. One variator profile might be such that the belt is carried higher up the pulleys therefore a higher gear effect is produced. Another variator with a profile biased towards acceleration would have a profile that favours the belt into a tighter smaller configuration so lowering the gearing. By the way nearly all so-called peformance variators come supplied with a new variator boss that is reduced in length so allowing the variator faces to be closer together and raising the overall gearing.
In my time I have owned Honda c50,c70,c90,s50,ss50, Suzuki a50, ap50, Yamaha fs1e, townmate, Rd50, Mobylette 50, Raliegh 50, Puchmaxi, Piaggio zip50, Velo Solex. I have spent a lot of time on 50's doing a genuine 40MPH!!
If your unrestricted 50 cannot achieve 40mph then there is something wrong with it.