I have always been a suzuki fan when it comes to street bikes. but if it was my choice, i would like to see a raptor with the new 09+ R1 motor. it has a new crank design and fireing order. it makes gobs of low end power and awesome top end power. its just a monster everywhere!.... i think it would be an awesome motor for a quad.
a busa quad would be freaking crazy fast too. i just assume the size of the motor is a little bigger then an r1 motor, so would cause for more fabrication. I have a built 05 gsxr750 motor that im debating putting in a quad or building a single seater buggy for. good luck with all your builds!