Unsolicited babble:
Clutch probably is not frozen, and you probably already know that by now. Cold oil viscosity can hold the plates together preventing the clutch from releasing until the oil warms up. You can use ATF, which should let the plates separate easier. Caveat is the clutch center could need to be replaced. It takes me two kicks on the kick start with the clutch lever in to free my clutch when stone cold, unless it sits for a couple of weeks, then it take 4 kicks. (Different Norton clutch though)
That motor sounds about right to me. You could rebuild it and it might rattle quieter if that is what you are looking for. I wouldn't take it apart myself, because it sounds fine, and you could ride it while considering how you would rebuild it. They aren't really that much fun to tear down for no reason. The motors are not fragile even though they can be noisy.
Ride it 50 miles, and change the engine oil and filter and gearbox oil while hot, and do it again. Then go to regular 1500-2000K oil and filter changes. I run 20-50wt with STP in mine for the zinc. Also quiets it down some.
Get out the steel wool and see if you can knock off some of the rust. Restoring one to perfection will cost more than a new bike before all is said and done.
BTW, I said the same thing about rebuilding the motor when I pulled my Norton out of the corner of the garage after a couple of decades of not riding it. Decided not to. It's a bit of a rattler, but it runs fine. Still gets me plenty of thumbs up when I'm out riding it. Well, I think they are thumbs up. I suppose they could be the middle finger.