I bought my bike from a friend in '79 who couldn't keep it running. Every single thing has been rebuilt or reworked. Yes, it has matching numbers... (woop-de-doo, not my thing at all...) Anyway, it may not be a bitsa because I started with a non-runner, with matching numbers, but it's by not even close to stock. It's the most ridable commando I could make it into for my needs.
One thing a bitsa affords you is chosing the better parts when possible. My '70 model had a flat headsteady, which broke because they aren't up to the job. Later models of commando have a box headsteady which are much stouter and don't break. You can push the envelope further regarding headsteadies and get a production racer headsteady or an aftermarket "ball joint" type headsteady which are the best choices... A lot of parts have these kinds of choices.
So since you are on the norton learning curve, you shouldn't jump at parts being offered to you until you find out if you are buying a part that norton modified on later models... There's a lot of this kind of thing. Sometimes parts that are cheap are parts guys are selling because they upgraded to the less flawed part.
Many norton owners swap out to lighter weight and wider aluminum rims to use a more modern tire profile more effectively. You get more rubber on the road with less unsprung weight, so handling will improve. There's aftermarket brake upgrades too. Since you are starting without all these things, you can go straight for the best stuff if that's what you want to do.
For commandos, the list of parts that can be grouped as either "original", "improved", or " aftermarket vastly improved" is a pretty long list. There's a few parts like the flat headsteady that should go right in the trash bin/recycle bin, but you will see them for sale... There's probably a few others out there which would be good to pass on in lieu of the later model part.
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Regarding your second post, my bike was a roadster originally. When fuel was leaded, I could stop anywhere and get good fuel, then it became unleaded, and further on it's ethanol laced. I transformed my roadster into an interstate so I can go to a dedicated non-ethanol gas station near me and have a wider range of travel with the fuel I prefer in my bike... It may not look as sexy as a roadster, but it's my frankenstein bike... Of course it looks horrible..