I've had it done by Fred @ Fair Spares for Ms Peel with unlimited time and expensive account and then years later on factory Trixie who just gets what can get by with so put cig foil back up in 0 ring groove then light greased the shaft the put RTV in groove and oil ring, stuck it together and didn't touch it for a day. So far with equal mileage on both fixes both seal about as well as the other, only mild weep misting if run hard and long or pretty fast. No drips no more. Was slightly disapointed the machined in seal didn't perfectly seal, so glad to DIY match it. Better crafters might find a better oil ring or solution that don't require sending off to alter another Norton item.
Trixie has about 7000 miles on new cam oll ring seal and just yesterday noticed a drip hit floor from it, ugh. Had a dry m/c lever seal too for a few month and this am after putting on tank after head re check, found the floor splattered and half empty of fluid. I'd not touched brake nor moved Trixie since New Yrs eve. Clean machine is rather a short lived nebulous sliding scale state. Just enough to smell right and stifle rust is about optimal.