Sweet! Very nicely done; it looks factory made!
The foam piece is a 3"X4"X1/4" sheet of medium density open cell polyurethane foam and, as suggested above, something usable can probably be obtained from your local garden or farm supply store. It should be dipped in ATF and wrung dry, just like the filter for your mower. It's important because it filters oil vapors from the blow-by that would cause the engine to run lean, smoke, and misfire. I don't have a pic of the filter itself, but here's one with it installed in the CB550 version of what you're working on, just so you can get an idea of what it looks like
Two other important parts in this setup are the drain hoses (18 & 22 ), especially #22. It drains condensation from the airbox, but has a special tip on the end that keeps extraneous air from entering the scavenging system. Be sure the hose either has the special tip, or rig some type of plug that can be removed occasionally for draining purposes.
Glad to see you're being so meticulous in restoring your bike; it seems like they always run better in stock configuration (duh!).