I am just a little confused.
What is it exactly that you can not find ?
There are 3 air filters for 18Hp magnum engines
A thin 7" diameter one ( element available in 4 different heights ) ring element
2 tall 6" diameter cylinder ones .
Oregon, Stens, Prime Line & Rotary all do after market elements for these.
Add to that there are a stack of similar sized elements for other brand that will fit onto the plate.
https://www.stens.com/brands/Kohler/part-category/Air-Filters-~-4~Cycle?show=48&keywords=magnum%20air%20filter