The engine on the YZ450 was redesigned for 06, so your engine is different. That said, anything is possible. My feeling is that the motor mounts might not line up, but you could cut and weld them and make it fit. Your swingarm might work because the swingarm bearings and spacers are the same between the two years I believe. You probably can use your front end, but will probably have to grind the steering stops a little to make it clear the frame. You would need some sort of oil tank. My thoughts are either buy one from a newer YZ250F, or rig up an oil cooler behind one radiator and use that as the oil tank also (you would have to have a way of draining the oil cooler when you do oil changes). The radiators won't fit, but if you are good at welding you can cut off the mounting tabs and weld them on in the right places. If all of these things work, then you would have to get a new subframe, gas tank, seat, plastic, and airbox. Looking at the price of all these parts used on ebay, it would still cost quite a bit more than to just sell your bike and buy a clean used 06. You have to remember, the newer bike will have better suspension
and newer parts.