An increase in power output at higher RPM than standard will give higher top-speed directly in proportion to the RPM increase (when standard gearing is used). In general, standard motors run at around 7,000rpm. The SP3 exhausts run at around 750-1000rpm higher, and the handmade exhausts run at around 1750-2000rpm higher.
The use of an exhaust on an otherwise standard unrestricted motor improves acceleration and top speed.

2. Cylinder Capacity - Use of a Larger Cylinder Kit
There are several manufactures of kits; Malossi, Polini, DR, Eurocilindro, to name a few. The Road kits from these manufacturers can be used with the standard head or with special cylinder heads that give greater performance. These kits are normally cast iron cylinders with aluminium heads, and run at the normal RPM for the motor or slightly higher.
Malossi, Polini and Eurocilindro do aluminium kits with Nicosil-plated bores that give more power and torque. They are normally supplied with a cylinder head and are in a higher state of tune than the Road motors, but are extremely reliable due to the heat-dissipating quality of the cylinder's construction.
There are also Race-type cylinders available that Malossi, Polini and Eurocilindro sell that have extremely high power outputs but are not suitable for Road use because of the high maintenance required to sustain reliability.

All recent 50cc Automatic Scooters are restricted to 30mph. This can be done in several ways:

Electronically, to limit maximum engine RPM and hence speed.
Mechanically, to limit gearing increases on the transmission pulleys.
By using restrictors in the exhaust and inlet system.

Also, some manufacturers use a combination of all these methods.
The following tune-up and performance-increasing modifications can only obtain the best results when the manufacturers restrictions are removed.
There are four main areas for improving performance:

Exhaust
Cylinder capacity
Gearing increases
Carburettor improvements

(Listed in the logical order for modifying parts.)
1. Exhaust
There are several types of exhaust available:

Those that give a small improvement at the standard engine RPM, i.e. a Sito Plus
Those that give an increase of approximately 20% but at higher than standard RPM and need transmission adjustments to optimise the performance, i.e. Leo Vinci SP3
Pure expansion chambers that give around 50% increase to power but at a higher RPM and require transmission adjustments and sometimes clutch adjustments, i.e. Leo Vinci handmade exhausts