@calum I'm not fan of 4-strokes either, i love the crisp sound of a 2-stroke motorcross over anything, the pipes looks 10 times better than the 4strokers aswell. 450's are sick tho, and in fact for mx the 4-strokes have a benefit because of the powerband being so consistent in a way.
My friend put a leovince exhaust on his mt125, it looked a lot slimmer and sounded better than the stock one. Sounds nice in the beginning but after listening to it for a while it turns into being annoying :P.