try an aftermarket airfilter.
the term rejeting refers to adjusting your carburetor so you are getting an equal air fuel mixture. after putting mods on your bike you will probally need to do this.
once you get used to it, it is very simple. look around in some forums and search for "jettin" i'm sure there will be more than enough info . :)
by the way, if you want you xr80 to SCREAM you could also play with the gearing. i forget what stock gearing is, but more teeth on the rear will give you more torque and less top-end. the opposite goes for the front, the smaller in the front will give more pull. changing the front will give you the biggest change over all. i think like one tooth on the front equals about 4 and a half in the rear.