Whatever you do, don’t mount your seat with rubber, you’re the most important part of the handling equation! You don’t want the seat moving around. A fully padded seat or inserts is an option though.

Vibrations are just part of the game but I’d start with these 3 companies and follow their advice:

http://www.swedetechracing.com/parts-accessories.html

http://sharkshifter.com/zc/index.php

http://www.musgraveracing.com/shopping/index.php?main_page=index&cPath=2

You also have CRG USA right in your backyard.

MX bikes have suspension and padded seats.  Its pretty much a solid path from the motor to your body in a kart, which has neither.  Isolating the pipe won’t do much as the motor itself is mounted solid to the frame, which mounted solid to the seat. A Tillet padded seat (Model T7, I think) is an option if it’s really driving you nuts and you can find one that fits.  Believe it or not, a TaG kart with a Leopard motor is even worse.  You’ll get used to it.