I do not advocate ruining vintage valve covers, but I would think there is a tasteful way to seal those vents that are open to road grime and underhood crud and then put a filtered breather cap on. If you did it on the inside, you could do it in such a way that nobody would be able to tell.
You would see engine longevity improve for sure. The same engines only filtered about 18% of the oil. No wonder they were susceptible to top end oil sludge/blockage.
