I will not argue with the "Stock is good enough" crowd, or the "Factory knew best" as if it works for you, then thats all that matters. But Trev is correct, There is a lot on these bikes that can be improved. Breather is one of them.
No lack of testing on other motorcycles and cars and science and technology have evolved since the 1950s,, Point in case, what was thought to be ideal Port and Polish techniques on intakes and cyl heads has changed 180 degrees. (Smooth & polished is not the ideal anymore)
Many other brands tend to have their own ideas (Dont go arguing with the Norton nutters!) But here is a design that has proven to be ideal. Buell came out with it on production machines and most modern brands now use a variation or something similar, Although, MOST run the venting INTO the intake as this is now mandated in most jurisdictions. I have updated my older V twins to this Buell type spec with their PCV valves. That Eric sure did know a thing or 2 about performance engineering.
In high RPM applications, there is proven DYNO verified performance gains with a driven air pump evacuation system, Not totally practical on a old BSA... but the test results are out there if you care to review..With such a small engine a pump design would benefit it, But the HP & Torque would not be that big of a number but the leakage problems would go away for sure..
Here is a typical Auto system... See:
https://www.jegs.com/i/Moroso/710/25900/10002/-1 (no pump)
https://www.ebay.com/p/Moroso-22640K-Original-3-Vane-Vacuum-Pump-Kit/15017008969?iid=172256694429&chn=ps (With vacuum pump)
I have seen over the years many dyno tests and race car engines can see between 15hp to as much as 40 HP using a vacuum pump design,
Here is a Big Block V8 test (Many youtube videos out there as well)
See:
https://www.hotrod.com/articles/ccrp-9903-moroso-vacuum-pump-test/" Peak to peak, the pump in its as-run configuration on this engine was worth about 14 lb-ft of torque and 10 hp over the no-system configuration. It was also up about 17 lb-ft and 6 hp compared to the exhaust evacuation setup."
More on the tech,,,,See:
https://www.dragzine.com/tech-stories/tech-how-external-vacuum-pumps-free-up-horsepower/