There's not much of an official swap at the Scramble, but there's always a few guys selling spare parts. One guy had an entire truck load full of minibikes at the first Scramble. You never know what will show up. This year the race will be July 25.
The Iron Invasion (October, Woodstock) has a good-sized swap for car parts, but has a minibike ride so you may find parts, and Vintage Torque Fest (May, Dubuque, IA) is run by the same people, so there's also a minibike ride and swap, just nothing minibike/go kart specific.