I think your best bet is going to be to piece together a scooter to get the biggest parts. 125mm wheels are currently the largest wheels out there for typical scooters, but not all decks and forks are compatible with them. In fact, most are not compatible. As for other large parts, the Phoenix Stan Smirnoff deck was one of the largest decks available, but I'm not sure if that is still the case. Phoenix was also making larger Klaar XL bars.