All the ones I have seen are painted and not anodised - so a good paint stripper is what is needed - the stuff you buy from a DIY store is not very good these days dues to changes in the law regarding what they can put in there.
I just buy neat dichloromethane (also known as DCM or methylene chloride) - that’s the good stuff they took out of paint stripper years ago.
5 litres lasts for years, and you can use it time and time again.
Also bear in mind that the bare aluminium ones can be purchased from Andover:
Motorcycle parts and spares for Andover Norton motorbikes: Andover Norton manufactures and supplies all Factory Spare Parts for Norton motorcycles built before 2002.
andover-norton.co.uk