Congrats on finding what looks like a fairly unmolested bike. As others mentioned, there was an era of engines that had bearing failures that almost killed Norton before they actually did croak in 75/76 depending on what you read and where. There is a lot if info out there on that but essentially as the pistons go up and down at the same time it creates crank whip and the roller bearings ate into the race. They fixed it by using a barrel shaped roller that was more forgiving to the whip. Not a great selling point for them. There's quite a bit of information on-line about it but disassembling the engine and replacing the bearing(s) is not a job for the not mechanically inclined. There are a lot of Norton's out there with broken fins on the head because people didn't know there was a hidden bolt between the exhaust rocker cages. Thereupon the prybar came out and the rest is history. That's just one of the many things that defy your ability to understand British engineering. I've seen somewhere, probably on this site but it might have been a vendor, where they have a description of what engine numbers were affected. There is help out there and wisdom and also some stuff on YouTube. Resist the urge to force anything.
That being said I'm no authority on anything but I suspect you will hate your fiberglass tank before long. I've had two small Hi-Rider/S tanks on my latest bike, both meticulously cleaned and sealed with two of the major tank sealers recommended on this site and others only to have the ethanol leach out resins in the fiberglass and foul the carbs, valves, and pistons. The Amal pilot jets are notorious for fouling from that, causing afterfire and poor or non-existant idling. There is no "ethanol free" fuel anywhere within 100 miles of where I live although you can Google it for your state. I've purchased two metal tanks from India, the last being what they describe as an "American Seller" only to be gobsmacked at how poorly these are made despite looking good aesthetically. I recently found a factory tank on EBay and will be working that back to an acceptable condition as time allows. So anyhow, there will be times when you might question your sanity and might wonder if you're in over your head but those of us that love Norton understand the allure despite all that.
One final comment, I got my last Norton (this is my third) about five years ago. The guy I bought it from got it from his buddy and he didn't know much but he was adamant that all that was ever used in it was RedLine oil. I kind of thought well whatever but when I pulled the head and barrel on mine to remove the leached out resin that fouled everything, the pistons were stock and the barrel true and it made a believer out of me.
