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I had a 2002 Yamaha BWS scooter. Bought is used. It would do 59 km/h flat out, and got 90mpg. I put new rollers (stock) and belt (Kevlar) in it and the it would do 66 km/h and 100 mpg. Finally, I put an Athena aluminum 70cc cylinder on it and higher final drive gearing. Then it would do 90 km/h, 100 km/h if I was in a full tuck, all with the stock exhaust and close to the same engine revs as stock. And, strangely enough, 120 mpg.
Most "modern" scooters (BWS, Vino, Jazz, Ruckus, etc) are limited by gearing. Once they reach ~ 50 km/h, the CVT is at the fastest ratio. And when it reaches its top speed of 60-70 km/h, depending on the bike, the engine RPM is above the power band and simply can't push anymore. I did try upping my final drive gearing before the 70cc upgrade, and this added about 10 km/h top speed. It did accelerate the same as the CVT took care of that. However, with the 50cc engine it was horrible from a stop. I was scared that I was going to smoke my centrifugal clutch every time I took off. Higher gearing needs 70cc. And you can't gear higher by changing rollers.
The other option is to put an aftermarket exhaust on. It will raise the powerband of the engine, thus raising your top speed. Most pipes come with lighter rollers to tune the CVT for the higher RPM powerband. It is the cheaper option than gearing and 70cc, but higher revs and loud exhaust at top speed would probably just annoy everyone around you.
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