I would start with, "What does a OEM Commando in good condition need to operate easily/safely on today's roads?" Frankly, if you started with a "new" Norton Commando as it was delivered in 197x, the only thing I think it NEEDS as opposed to "cool stuff I want to buy" is better front brakes.
Admittedly a lot of stuff that we install nowadays is convenience related - less maintenance. E-ignition, for example. Not that that's a bad thing but there is a difference between what you NEED to do as opposed to what you want to do.
Don't misunderstand, I am not against buying new parts. Among other things that I can't think of, my Commando has a Trispark ignition, an Alton E-start, a CNW Brembo Master Cylinder, Swingarm Clamps, a high-zoot crankcase vent, two Amal Premiers to replace the orignal Amals, the Fauth front fork mod, the CNW 520 chain conversion, new wiring harness, SS brake and oil lines, etc. Heck, I just received a (gorgeous) Rear Set from CNW and a pair of Aluminum-body Ikon shocks. The bike doesn't NEED them to function but they sure look cool! They look SO good it might require me to repaint the bike!
Of course you need all the normal replacement parts to get the everything in good, original condition - Iso rubbers, maybe a top-end engine rebuild (rings/valves/guides/seals), tires, spark plugs/wires, wiring harness, new mufflers and maybe pipes, etc, etc, etc. But that's just "normal" maintenance though it may have been deferred for 40+ years, making it rather expensive all at once.