On most engines, I find air filters to be pretty overrated. As long as you're not running in a sandy or dusty environment you're not likely to suck up enough crap to kill the engine a whole lot faster than you normally would have. You're definitely not helping the reliability but you're not hurting it as badly as some would say. 2 strokes in particular don't seem to need much air filtration on account of not having a valve train.
I've got a mudding tractor with a 13.5 horsepower briggs on it, it's gone through dozens of waist high mud pits with the only air filter being a sock attached to the end of the snorkel with a hose clamp. It looks trashy but it's cheap and gets the job done, and is very easy to clean it once it gets too caked up to work right. My MAB has a velocity stack with a bit of foam inside, so not a whole lot of filtration is happening there either.