Go to partstree:
http://www.partstree.com/parts/cub-c...-669970-below/
#21 is the spacer/seal.
#24 is either a umbrella poppet valve or a flat metal reed, they need to be need intact you will also need 2 #17 gaskets.
You can make them or purchase, don't use sillycone.
#22 is a little piece of hogs hair filter, it should be there also.
if missing you can use a piece of chore girl/boy copper scouring pad.
Just follow the tube out of the air cleaner to the cover/breather.
it takes a little 7/16" wrench or socket.
pretty simple.