Hi Team,
when I did the Bosses Thirty we were missing one of those non-return breather valve caps.
Cheap replica was to find a brass hose barb fitting from the local plumbing store that had a body, etc. almost identical in size and shape to an original.
I machined the middle barbs of the body, but, left the top ring to replicate the shape of the area where the breather cap and tip goes. I machined a replica cap/top and breather outlet, and some baffles for inside--no valve-- and silver soldered them all together, result a pair of fairly genuine looking replacements that I doubt anyone has noticed yet after about 16 years.
I did make two replicas and the boss kept the original in case some funds were found to purchase another.

Looking in my Parts Books :- 2 drilled pipe plugs, S-818 or 2 x 3476-A, what ever's, could be used.
Cheers,
Eddie B.