I run a foam Kawasaki filter that I got on ebay for $10. I also have a cheap foam no-name filter that was about the same price - I had to trim it to fit but it worked fine too. It's a spare because I would rather run something that came out of a "Kawasaki" box, all other things being equal
I have always been a fan of running factory-spec air filters. If it's still in good shape and needs oil, error on the side of oiling it
lightly. Too much happens fast.