The Mk I valve guides have short, rounded tops which makes sealing difficult.
The Mk II had taller guides with cylindrical sides and an 'oil scraper' conical top face. Also there was a steel top hat sleeve that fitted inside the inner valve spring and slid nicely over the cylindrical guide - keeping oil mist and liquid away from the valve stem.
Changing to Mk II guides might be an answer (if they fit - I didn't do this and don't know the respective dimensions). Alternatively, getting new guides turned-up with a top that allows the rubber 'valve guide seals' that one gets in car engines to be fitted would be a better option.
The last car top-end I did was a Toyota RAV4 (2.2 litre twin-cam common-rail diesel). The rubber seals with lip springs just clipped over the end of the (16
) valve guides. Hand-lapping those tiny valves was a bit of a chore
I am not averse to modernising old machinery providing it doesn't detract from the overall look and character.
My avatar shows the late Len Rich in 1970 with the bike I now have - a 1958 Ariel VH