I found this (Uni Filter BF1 12" X 16" X 5/8" 65-PPI Green Fine Foamon) on Amazon and ordered some after seeing the Yanmar
filter foam disintegrating into the engine
intake. Granted, the boat
had been sitting in the yard for over a year, but still. I unsealed the new, spare, Yanmar
filter to install it, and the foam fell apart in my hand.
The foam sheet cost is around $13 and is washable and suited for the high heat generated by motocross bike engines etc. Your air intake filter is inside a steel
housing, so a steel
mesh wrapped hunk of foam , if flammable, would not have much luck burning inside it. I too attempted to find a K&N filter without success.