I know some repair shops almost force new replacement parts if it's an easy way out. Some of the old motors had their advantages, and you might want to search up someone who cares enough to bother. I have hardly worked on machines with the stronger clutch motors, just tried a few out, so I can't really recommend anything in particular. Perhaps you can find new motor behaving much better at a reasonable price.
In the past I have gone through a few weird things like this; I go and look for someone who are able and willing to take a look at my bike, it has a 5 speed hub that doesn't shift properly. The bike is less than a year old and not used much. I'm told it doesn't need anything, just wash the bike and oil wires and chain, to keep on using it as long as it runs and shifts. I'm told to buy a new bike or replacement wheel with a gear hub when needed, (there are 7 and 9 speeds hubs if I like an improvement). Very annoying thing, in the leaflet that came with the bike it said the hub needs service after a certain amount of kilometers. I end up going home, cleaned and oiled best I can, polish parts, go over everyting. I'm suggested all kinds of things by people around me; to add some oil to the inners of the hub where the shifter cable enters; to never use oil on greased hubs; to use WD40 to loosen up the old grease; never to use anything lighter than two stroke motor oil,... I go on for years like this with very conflicting advice every which way, and I'm really not getting anywhere. For some reason this bike was never thrown away, but over the years I had several bikes along side it.
Then I get an old 1950s bike I take a liking too, lots of work to tighten spokes and sort out the bottom crank bearings. I happen to find two repair guys who bothers to take a look at it. The gear hub has 3 speeds and an oil cap. I add oil and things shifts and run better than any of my gears have. The repairs and fix ups happen gradually and the bike firms up surprisingly well.
I'm thinking there's a chance my 5 speed hub can improve, by then it's been standing in a shed at the summer cottage for years and I need a bike there. I take it home, add a teflon enhanced oil to the hub. It does run and shift, but it turns out inner wire has a split wire inside the outer cable, not easily detected and probably has been like that for years. The wire gets replaced, along with new tires and inner tubes, the old bike turns out to be nicer than I ever could rememer it did. The gears behave flawlessly and still does.
To sum it up; if I just had come to the right guy who was willing to do the job, it all would have been sorted out years ago. There were nothing really wrong with the hubs or the parts to begin with, they needed oil, cleaning, a new gear wire, all just basic maintanance.
I have had this happen a few times, with bikes as well as a sewing machine. Now I only trust my self when it comes to judging what's worth a repair and what's not. I used to listen to the expert, but when you have met an older guy who could help but just didn't want to do the job; other guys who didn't want to work on anything but the bikes they had sold within the last three years,... I don't trust anybody anymore. I have become very stubborn and peristent if I have an old bike I like
An old item needs a bit of an investment to run again, a bit of time and energy, sometimes a bit of money in replacement parts. It can be well worth it, and I know you know this well. Some items I like to keep as original as possible. When all parts are there and they are repairable and working well, it's a shame to just to throw them out. Machines that's deliberately run hard and soon just exposed off it's different. Some are resilient enough to surprise.