As has been said you have a piece of familiy history. So I assume you want to 'keep it in the family'.
That said it is clear you want to make it run right now. I am not sure you will benefit much from that exercise, maybe, be patient. I hope you don't mind me suggesting you change your priority. I am sure you will do what you want anyway.
But it may be a ogod idea to decide what you want to end up with first. Let's say that might be a functioning and clean representation of what it has been since the early '70s without being a slave to what could be accomplished back then. This may have some market value, but most likely greater personal family value. The alternative of returning it to an original looking fatory bike would be pretty much more than it is worth.
So, once you decide the end game you can strip and refurbish the bits you want to refurbish and change the bits that need it or are maybe not done as well as you would like. This list will include a range of consumables, chains, tyres, various rubbers etc. Maybe wheels/brakes/forks, and I am guessing a seat.
Probably better not to run that engine full of sludge and goo anyway. If you want to know the condition of it it will be better to strip it. So I suggest you strip, clean, replace what is needed, maybe not much, reassemble with care. Choose if this is to a show standard or just nice clean stuff. Improving the performance would not seem a priority at all.
You are going to learn a huge amount from that excercise alone including the research. You can't know what Grandpa was thinking, wanted or did, or how well it worked. So somewhere along the line you need to make this thing yours anyway.
Check the fuel tank, is it viable for modern fuel? Repair or replace, either way custom work. Check out and clean that oil tank, or again replace.
Then. Carbs of choice, clean and ready to go. Plan and install a more modern solution for wiring and ignition, probably including a small battery ($$), but self generating solutions do exist from magnetos ($$$$) to PVL ($$$). Saves messing with what you have, which on anybody's part is just guesswork, some pretty educated, but guesswork. And in the end it may do more harm to the motor than being patient.
In the end it will run and you can ride it. Decide if you like that or just want to own and show it. It is a period piece, genuine history and family history. The Commando world might want another 1968 Fastback, but it doesn't need one.
Or you could just sell it and someone will drag the motor/gearbox/primary out, build something they claim is an original Fastback and ask silly money for their 'fake'.
Good luck choosing your path. And I don't mind at all if you just ignore me, enjoy your Norton.
(circa 1975)
