Putting a bigger carb on without porting the intake and the jug will get you nowhere. My stock V1 was the 19mph model, and the intake port was smaller than a dime.
After porting, 16mm carb, home built pipe, and a 36t sprocket, it did 40mph, but took 1/2 mile to get there. You could walk faster than it off the line.
Best advice is get a polini kit. Instant 5hp, according to specs. A small price to pay for more power.