We've ridden this subject into the ground, No they don't need one except in very dry hot conditions riding same direction as the wind. The amount of oil in circulation in Norton twin is not enough to really cool engine, maybe like 6% of total heat flow is via the oil, so only reason to cool it is if oil got much over 200' F which is the heat level needed to get the dry start protection of the famous zinc/phos ZDDP. Take rather over 160' F to cook off combustion moisture too.
If ya do, use a thermostat bypass and maybe even a leather cover for cold times like the HD twin guys use to keep oil temps up close to 200' F. If you can get your Norton oil in tank to read over 190' F in public I will shake you hand on such hard hearted WOT risk taking for over 50 miles to do so.