Some gearbox sprockets are held in place by a circlip, as there is relatively little sideways load, that may be a viable option, just get an appropriate groove machined into the output shaft.

Had the bike a couple of weeks now, it replaced a Triumph 900 Sprint Sport that I just couldn't live with, much too tall in the saddle for my 29 inch legs.  I think they must have shrunk because I didn't have a problem with  with a Honda Africa Twin some years back.  But I've found posting pics on here is bit hit and miss, they showed them the correct way up on the mobile site but upside down on the full site!

And I know velocette is spelt incorrectly but I blame the iPad.