Robin OEM engine filter replacement:
This was done to avoid having to purchase expense OEM filters over the web. You should be able to save a few bucks and go to the local tractor supply store and find a Kohler filter to use. I believe this should work on the 18, 22 and 25 hp versions, but the one in the pictures is a PT422. I based the following on an email from forum member washita although I did my mine slightly different, but the idea is the same... remove the metal cover from the OEM and use it with a more readily available filter element.
First, separate the paper element from the stock filter metal plate. There will likely be some rubber residue along the edge of the plate, so I just ran a wide blade screw driver along it to lift it up. Some of it would pull away in chunks making it easier. I don't think it has to be perfectly clean, but it needs to make a good seal with the top of the replacement filter. Be careful not to jam the tool into your hand while peeling up the residue, or cut your fingers on the wire mesh surrounding the paper element.
Washita suggested I use air filter #47 883 03-s1 for Kohler Command CH18-23 hp and CV17-23 hp engines. I used a slightly different model as seen in the following picture since it was all Tractor Supply Co had yesterday. The other model numbers may or may not be a better fit. I noticed the one I used (4708303) mated with the metal plate well, but was a tight fit onto the carburetor housing. The cost was $12.49 and included a prefilter.
You'll need to push down with enough even pressure to get the filter onto the carb housing without crushing it. This ensures a good seal, and allows the nut to reach the the threads when tightening it all down... but more on that later.
Here's a pic showing the metal plate over the installed filter, along with the prefilter. The nut is screwed on just to show how the metal plate makes a good seal with the new filter.
Since the filter I purchased had a slightly smaller inside diameter, I needed to push the element down onto the carb body with some force to get it to sit lower. Even after doing this, the filter was a bit higher and and wouldn't allow the nut to reach the threads when everything was installed, so I had to remove some material on the bench grinder.
Here's the last pic with the cover in place. The total time to do the mod was about 15 mins. Hope this helps someone else too.