Air filters are designed to filter contaminates from the air going through to the carburetor, get the wrong one and not enough air will get through to the engine making it stall or if it's not fine enough then it won't filter and you get a dirty carburetor.
I'm sure it's not an exact science but you'll know if it's wrong, I used to have to change my air filter on my boxxer 421 regularly I think my van was too small and it got too 'sooty' with carbon deposits, many a time I would have to take the air filter out to get the TM going and run without it, my water cooled Boxxer 427 is LPG'd and my van is far cleaner inside also the water cooled engines don't labour so much.
Shaun