I think this guy is trying to keep the bike looking stock, yet trying to integrate the Moto-GP style exhaust in there without attracting to much visual attention. When I first glanced at the video, I did not notice the exhaust until a few seconds in, so I think he somewhat succeeded.
Cartman has done some great songs over the years. Although my favourite has to be this:-
[ame]http://www.youtube.com/watch?v=u9u_u7sz4to[/ame]
I have a flash animation of it at home made by the South Park guys. I'm in Ireland for a week, but if someone reminds me later on I'll upload it somewhere on the net as they seem to have mostly disappeared.