Yes AV8TOR, I have fitted an air filter. You can see it in the attached picture. It consists of a stainless steel coffee or tea strainer covered with a self made foam sock. I made the foam sock by hot wire cutting 10mm foam sheets from some scrap. The ends are bevelled and glued with an acrylic glue. The round end was hand cut and glued in. Seems to work fine.
I have never run the engine without at least the fine stainless mesh strainer on, so don't know if the noise level would be higher with it off or not.