2002 450FE back firing

Ive noticed lately my bike backfires more and more. I have read that my old rusty exhaust could be the culprit. If i turn the idle up a little it doesn’t do it as much. If it is the muffler, any suggestions on a new one? I saw how expensive OEM ones are. I use this bike for woods and mud so I’m not looking to restore her. Any advise would be great. Thanks guys.