Uhhh to put it simple, none.
The Helix or Morphous I think have the biggest aftermarket...but almost everything cool for them you have to get from Japan...most of the time that means $$$$. And both have really big asses, which means its hard to make an extension work.
The Big Ruckus has decent aftermarket support in the US, but even then theres hardly anything to visually mod. And good luck finding one, let alone one for less than $5k.
Honda Reflexes are cool, but I don't think there's much US aftermarket support...
You can lower or make a stretch kit for just about anything... but the degree of fabrication can vary. I'm sure I'm missing some big scooters.
FACT: I'm gonna start a new thread called: "NADSCAR - we don't race in the rain..so fuck you"
1980 Yamaha XS400
1985 Chevy M1009 CUCV
1985 VW Vanagon