I have just looked through all of my Trusty Steed records and can’t find any mention of that engine number. Don’t be surprised, though, because after March 1953 they only recorded the engine type, in your case Norton, and engine numbers only appear very few times.
Similarly, the registration number will only be recorded in the factory records if it was sold directly to an end-user. Normally they were sold to dealers who would then sell them to their customers and so the registration was carried out by the dealer, not the factory.
Sorry that I couldn’t be more help, but sometimes it works out that way. Now that you’ve finished it, how about some pictures of it?