Once upon a time I was in the air filter oil and air filter business.
Please do not use K&N type filters on a dirt bike that you care about. Yes they flow more air. But they do allow larger particles through. Dirty air is bad enough for a 4stroke. But it's twice as bad for a two stroke since the dirty air is carrying your bottom end lubricant. Just don't risk it.
I would suggest you be very picky about using the very best filters. A dual density foam like Twin Air is the best, IMHO. Get at least two so that you can have a clean one ready for speedy filter changes. Having two will help you change filters more often. I try to change mine every single ride, even if it's only slightly dusty out.
The brand I distributed was No Toil. Now there are other products with similar easy clean-up. Many folks are happy cleaning filters the old fashioned solvent way. Whatever works for you is the way you should go. Just make sure you do it often. It is at least as important as changing your gear oil.
...slowly wandering off topic...
My relationship with No-Toil ended badly. And I have some hard feelings about Bob.
But I stand by the product I was peddling. The oil works great and the filters are high quality (very similar to a Twin-Air). I made some good friends and learned some hard lessons durring those years.
But here's the cool part
...
There I was, it's 2001, and I'm quite suddenly in the business, driving from dealer to dealer trying to set up a display, and exude credibility. They ask me what I ride, and I have to respond "a 1987 KDX200 that I got for free because it was a basket case". While a Kawasaki dealer might get a warm glow from that answer, it was not really working with universal appeal.
Jim Cook had the answer, a leftover xc300. I told Jim I didn't know if I could handle a 300. He assured me it's the easiest one to ride. I probably should have ordered three or four extra rear fenders. But I finally got the hang of it, and never looked back. I love my "What What?".
It's true. I got my Gas Gas largely so I could be "brand neutral" at Japanese dealerships. And the Orange dealers only gave me knowing nods of approval.
Somebody said that owning and maintaining a Ducati is good for one's soul. Perhaps the same could be said of the mighty Gas Gas.
We are lucky.