Sorry I didn't make myself clear. Your carb and fuel pump works by vacuum created by the engine when running. The faster it runs creates more vacuum and raises the slide more via the diaphragm allowing more fuel/air to flow thus increasing RPMS and so on. That inlet simply lets outside air in below the diaphragm as part of the vacuum system. The filter simply keeps dust and dirt from getting into the carb and gumming up the works.
ANY kind of filter that will allow air to flow in and keep dirt out of the inlet should work just fine.
I'm sure the machine will run OK for a while without a filter BUT dirt would very soon clog up the carb.