And after time they leak around the oil scrols because the hole has enlarged and the scrol is no longer able to pump the oil back
They leaked oil from new and period publications were not afraid to state that
So you can modify the decompressor , put sealed bearings in the gearbox ( oil will still leak out the breather )replace the cork tube replace the pushrod cork but it will still leak from the kick start and from the gear change and from the clutch push rod because the corks are not constrained so that can be compresses like a gland
spend forever knocking the bolt holes in a tin primary flush with the surface again replace the felt behind the sliding plate ever 12 months but if you use it it will leak oil because that was not considered a problem back in the 1940's, 50's, 60's ( and in the UK 70's & 80's )and none of the sealing systems are oil tight .
The breather will blow a lot of oil mist out onto the chain, in fact that was a design feature
Now If i was doing a bike that was leak free when new then that is a different matter