Based on 10 new maxi scooters since 1990, all but 2 purchased in USA----no dealer stocks scooter parts to any degree! Some big, old, Honda dealers may have drive belt for SilverWing. Travels for me, 2014---49 states, Burgman 400 and Suzuki TU 250----this year may end up riding to 48 states. Good dealers really hard to find if you need them, Kymco has far amount of dealers if you can find them....but I think their interests are ATV.
Scooter West in CA probably has everything you need for Vespa.
Travel with scooter in US= you are on your own figuring out how to keep going---I try to start long trips with fresh tires, carry tools to make adjustments, tire pump/tire repair kit, scooter in good repair----really, only Harley has enough dealers, and probably some parts/tires in stock for the big twins.