This thing is only fifty+ years old?
Why doesn't that valve train look like an ad for Pennzoil Ultra?
Seriously, considering the oils this thing would have had for most of its working life, it looks pretty good up there.
Ignore the inside of the valve cover.
You can easily clean that.
As for the rest, any current oil changed on a reasonable interval will make it cleaner over time.
I don't know the maintenance or service history of this engine, but if we could go back in time, I wouldn't be surprised to see that it was worse in 1975 than it is today.
Oils are much better now, and are certainly better than what the engine would have gotten in its early working life.
Relax, run some short changes with any current 10W-40 while you work the machine, and then take the valve cover off a couple of years from now and post the pictures here.
If you want fast clean, there are various solvent products that might give it, but plain old oil will also make this engine cleaner over time.