The crankcase ventilation system on B18 and B20 motors came in a variety of flavors. My 122S happens sport a breather built into the oil filler cap which is supposed to draw fresh air in though the air filters.
Unfortunately, the non-OEM air filters on my motor don't have a hookup for the oil cap breather, so I decided to add a filter directly on the oil cap to prevent airborne nastiness from getting sucked into the valve train. Besides, there appear to be some advantages to disconnecting the oil cap-to-carb connection.
I initially used an air filter from a .21 R/C car engine, which worked perfectly well, but I couldn't resist picking up some "bling" the last time I was at the auto parts store (a K&N replica that set me back about $11).
Chrome makes you go faster, right?