I put the faster oil pump gears on an otherwise stock Norton twin about 20 years ago and it's present owner is still riding it around. I have seen other examples of people doing this mod with no problem too. The oiling system on the early Norton Dominator engines really is marginal, we know this because current builders of Norton performance twins even think that the late Commando oiling is marginal!
Not everything Norton did at the factory was correct. They made a few mistakes here and there over the years that can be corrected. My friend that told me to ditch the breather disk used to work right at the Norton factory in the late 50s/early 60s, then he was the head technician for the USA Norton distributor and head of it's racing team, so he certainly had information that trumped the factory service manuals and parts books.
That said, people rode many thousands of miles on Nortons as they were assembled from the factory, so factory service manuals are never going to get you into trouble.
When you rebuild your cylinder head note if it has scrolled or plain rocker arm spindles. The spindles should be tight in the head, and the head should be heated when the spindles are installed or removed. If you have scrolled spindles then you have to feed oil to them from the return line back to the oil tank, if the spindles are plain then you need a high-pressure feed to them from a banjo fitting and bolt that is underneath the oil relief valve in the timing cover. Getting the wrong oil feed to the wrong type of rocker spindles can cause Big Problems. Since your bike is an early model, it would have come with the scrolled rockers and a fitting on the return line to the oil tank to run a rubber hose up to the top of the head to feed them, unless of course someone swapped newer parts in at a later date.
Your engine originally came with low compression pistons, around 7.5:1 with dished tops. I see in the photos someone had installed flat-top pistons, which would boost the compression to about 9:1. This is okay as long as you use the ignition timing that was used on the Commando 750 which ran the flat-tops from the factory. When you set the timing try to err on the side of less ignition timing, that will help you out if you get some bad gas or are running in really hot weather. 29 Degrees is what famous Norton tuner Paul Dunstall recommended for Nortons with 9:1 compression, I would certainly not have any more advance than that.
The monobloc carbs have to be taken all apart and looked at very closely, every little passageway has to be gently and carefully cleaned out, never use any sort of bead blasting etc. on the carbs, this is hand work.
If the carbs are clean and installed properly, the only other thing that spoils idle quality is a bad ignition advance mechanism. They can get sticky or the springs can go bad and cause erratic timing at low rpms, so it is something to look at if you have any problems.
If you are mechanically inclined and have a clean work area, you should have fun taking the magneto carefully apart and cleaning it gently with a gentle solvent like kerosene etc. and compressed air. Watch out for the delicate shims that set end play, they are very thin and can be damaged. Put it back together with a very little bit of high-temp wheel bearing grease on the ball bearings, work in enough with your fingers to give them a small coating. Remember too much grease here will creep through the magneto and muck up your brushes and eventually stop it from working well or at all.
I think you should be pretty much set. Remember your best friends are information and patience. Let me know if you have any other questions or problems.
You are lucky to have got your hands on a Norton from the 60s, they are pretty legendary bikes and are rarer and a cut above their contemporaries. When it is running well you will be very impressed with it's torque and speed.
