These guys got a good point. I would go with the 450r carb, aftermarket exhaust, and filter. A huge difference can be made in a quad by moding the the suspension and gearing. You could go with some 450r shocks and drop a tooth on the front sprocket. A smaller front sprocket will make the quad a little slower but you will gain some torque, besides it only costs like 20 bucks so if ya dont like it your not out much $. Everyone says there is no relacement for displacement but I think suspension and gearing are just as important as displacement. Plus you keep the motor reliable.
Almost forgot get a good clutch kit. A aftermarket cdi will give you 1K more rpm thats about it. You can also run a aftermarket fan to keep the motor cool but Iam not to shure how that would work out with the mud on a ex.
Click on the link I posted earlier and scroll down to the bottom. There is a usefull cdi trick/info.