All Piaggio/Gilera’s come with a 'basic' CVT belt drive transmission and dry centrifugal clutch on the rear hub. Nothing unconventional or radical here.
Replacing the belt is a relatively straightforward job on all but the  500 models. On the 500’s the footboard body panel has to be removed to allow access to the transmission air cooler deflector plate which  has to be removed to get the main cover off. Additionally the Evo models require the clutch centre nut removing before the cover will come off.  The 500 SL has in internal clutch support brace and the centre nut is removed after the cover.
Once the cover is removed, it is simply a case of preventing the engine and rear axle from turning while you remove the centre nuts from the clutch and variator. Special tools are available for this, but if you use an air or electric impact gun with the correct socket, you usually  don’t need anything else. When both centre nuts are off, the variator and clutch assemblies will pull off complete with belt.
The belt is marked with direction of rotation which must be adhered to and you must not get oil or grease or scratches on the pulley faces or the new belt. Refit the pulleys and open up the rear outer half so the belt sits deeper then it is easier to nip up the front centre nut without trapping the belt.

Replacement tuned variators are available from Polini and Malossi and there is a new design from Spain called a Costa Transversal which uses ball ended cylinders running in a sequence of tubes rather than rollers and ramp plates. You can also get sliding weights as direct replacements for the stock rollers from Union Materials in Taiwan.
Belts will last up to 50% longer than Piaggios’ recommended service limits and rollers will easily last 2 belts. Keep an eye on your belt though if going long distances and it is past it’s service limit as they snap without much warning. One sign of imminent failure is a grey stringy  substance coming from the clutch air vent hole.