I've noticed that the latest "Desert" tires are coming out in 19", which for most folks should be fine. We are all running good H/D tubes....right?
I prefer the 18" set-up for desert racing due to the additional/taller sidewall: It's basically a second suspension and is more forgiving if you are out there in the rocks and square edges of the desert. Also, with good tubes and tire prep you can get away with very low/forgiving tire pressures. I've had NO-FLATS with 695 Deserts and H/D dunlop tubes. Sometimes we run "Slime" in our tubes for insurance (Vegas to Reno).
It seems that the Yong-Guns are learning this, too: The hot set-up on hard-packed rough/square-edged MX tracks is a 20" front and an 18" rear.
Maybe this means that there are more 18s coming?
If you've got a good 19" wheel, keep it. If it's toast and you've got the $: Build an 18" and enjoy the best fo both worlds.
BTW, there's an 18" Kawasaki wheel on some of the early-mid '90s parts listings. It's a KDX200 wheel that bolts right on the K5, if I'm not mistaken... I'll check next week and see if I can get the P/N. As I recall, it was pretty inexpensive. Also, some of my wheels are designated "99 KX250".
Best of luck,
Manny