From MopedWiki

Single-speed Ciao and TFR:

65/70mm pulley - 9,5x925mm

80mm pulley - 9,5X940mm

95mm pulley - 9,5x960mm

Variated Ciao, TFR, and Magnum:

90mm clutch pulley - 13x950mm

100mm clutch pulley - 13x975mm

Variated Bravo, Grande, and Citta:

90mm clutch pulley - 13x1025mm

100mm clutch pulley - 13x1050mm

Variated Si:

90mm clutch pulley - 13x1125mm

100mm clutch pulley - 13x1150mm

Gates ax35 or ax36(too long imo) for variated ciaos, if you are running a multivar douse the belt in belt dressing to be able to actually start the bike.

For a more complete list, including non-US single-speed models, go here.