The bezels, glass, and gaskets are available from a number of usual sources, Bernier Vintage, Accessory Mart/Domiracer, to name a couple. There are two rubber gaskets, a tapered one to fit into the bezel and a flat one to fit between the glass and the housing.
A buddy of mine made a slick tool from a plate and a couple of shafts and ball bearings to roll the new bezel on. Others have used a lathe and bearing. Barring that, you could very carefully use a hammer and a flat drift.
'73 MkV 750 Roadster
'73 850 Interstate
'75 850 cafe racer
some Nortons in boxes