I'm racking my brain trying to think of the model a guy in our scoot club had. Not a sento, but another 50cc 4T.
That thing was seriously slow. Every week he would show up at out ride night, with another mod to make it faster, but it never seemed to get any improvement.
CDI, Variator, Springs, rollers, clutch, exhaust - anything he bought and installed were really a waste of money.
But I am pretty sure he never increased the displacement. Nowdays, one can get a decent "big bore" kit, better head, and cam. I think those things are what you need to concentrate on.
As the saying goes - there is no replacement for displacement. Especially true for the scooter crowd!