needs to be that Inner Diameter on the lip of the breather, and short enough
to fit in the space behind the carb...
they do make some filters 'flat' like that. while they would technically flow similar amounts of air (had both styles of breather, air flow also has a lot to do with direction and air speed too. (graduated from Nashville Auto-Deisel College in '93 with a 3.6 gpa, 3.9 in carbs and fuel injection...) the foam dissipates the air coming into the carb venturi, stalling small amounts of airflow. the clamp ons will let the air make more smooth turns into the carb venturi, allowing for a more atomized fuel/air mixture. meaning more power, meaning more speed.
have a Cosmo/Avanti/Garelli so I don't have to worry about hitting the fender, just the frame.the space behind the carb