The Emgo shifter is a nice upgrade over the stock one on your bike. It appears that GG started putting the Emgo shifter on the new GG bikes. Many of the new GG parts can be retrofit back to the old bikes.
https://www.ebay.com/itm/Emgo-Forged...sAAOSwRXhcwxg5
I have a 2000 XC300 and it uses the old KX plastic and it does not have the airfilter door. The old blue plastic is getting hard to find. You can either try to find new blue plastic for your bike (which sounds like 2001 and newer with air filter door) or you can swap to red and white or you can change to a 1998 -2000 airbox and run the older KX plastic. You can buy new Polisport red and white plastic for the 2001-2006 bikes for about $200.
Do you need all of the blue plastic or just a couple pieces?