Hi Bignic, & welcome.
The Fastback is my favourite Commando style-wise. I had a 72 version many years ago. I hope you bring it back to life & discover motorcycle heaven.
For the chrome, I have found that "rust buster" ( otherwise known as rust converter, basically phosphoric acid) can work miracles in cleaning up rusty chrome. And if the result isn't satisfactory, it provides a stabilised base for painting.
That early frame is a worry. From what I've read about them, I'd rather not ride one. The bracing tube shown in Matt's pic. was added as an afterthought by Norton, so maybe you could have one added to yours. Just take note that the head steady attachment points are included in this tube.
I've just had a cross-tube added to the bottom frame rails of my 70 model to enable me to fit the late model side stand. I bought the 1" tubing from the guys I bought a box trailer from, and they did the welding too. They weld all day long, so I figure they know how to do it.
Apart from the engine, brakes, tyres,suspension, ignition, carbs that have been mentioned, there's also the isolastics to consider. They can adversely affect handling if deteriorated/badly adjusted. I recently replaced mine with the vernier adjustable type, and the old rubbers I took out were badly worn and perished.
Also, that head steady bracket was prone to cracking. My bike had already been fitted with the later box-section type long before I acquired it.
Oh I nearly forgot: the gearbox layshaft bearing has a reputation as a weak point. If it seizes it can lock up the gearbox and back wheel. Pulling the clutch in won't unlock the wheel. I replaced mine with a roller bearing.
Have fun.
How many winters do you have?
Cheers
Martin