Here's where I'd start...
First, pop the cap off the idle mixture screw and set it at about 2 turns out from fully seated. If you get one of those adjustable mixture needles, you'll want to add at least another turn to that due to the difference in taper. You may have to remove the cap covering the screw by drilling a small hole into it. You can insert a wood screw to grab it and remove it from there. ***Don't drill too deep, just through the cap. You don't want to mess up the adjustment screw.
Second, do the 22cent mod also called the "smile maker" by shimming the needle with two #4 washers - use the stock main jet.
It is up to you whether or not you want to drill the slide while your in there. 7/64" is a very common size...its probably not a good idea to go bigger.
As for the air box, I'd start by pulling the snorkel. If you find it is running rich later, you can always comeback and cut into the air box.
Also, I've been using the K&N filter for a while and though its a great filter, I'm going to be switching to Uni or Twin Air. It is just too easy for dirt to get around the filter if you mess up the application of sealing grease.
An option to the "smile maker" would be to add a KLX needle which is adjustable. I'm running the KLX needle on the 3rd groove from the top with a 145 main, IIRC. The taper is more narrow on the KLX needle so you have to drop some jet sizes. I'm running a bit rich, so I would suggest starting with a 140 main if you go this route. I'm running a free flowing header, with no air box mods, for comparison. I think I'd be happy with just pulling the snorkel and going with a 142.5 main.
Here's some sources for the parts mentioned as well as a couple of others for some extras you might want to consider while you have the carb off.
KLX Needle..........Schnitz Racing -
Adjustable Mixture Screw..........TPI -
Allen Head Carb Screws..........Eagle Mike
Alloy Starter Choke Plunger..........Stead Engineering -
Well, welcome to the forum and happy farkling. Let us know how it works out.