I've put a 2002 engine in my 2008 chassis when the 2008 engine selfdestructed. Engine fits right in, you need to swap the cdi also, since the 2003 uses the SEM ignition and the 2004 uses the kokusan brand ignition.
You can use one of the two yellow wires (these are uses for lights, horn,... on the 2003 chassis) wich come from the SEM stator to power the stock wiring loom for the newer chassis.
I've also swapped over the voltage regulator from the 2002 bike to power the lights on the 2008 chassis, couldn't get it to work through the stock 2008 regulator.
If you have a blown up motor, make sure you swap the 2004 carb, side cover and kickstarter over to the older bike.
If you have more questions, let me know!
