Like your idea of leaving it 'as original as possible'. While having
it work for today using mains electric power. Good for you.
Just a thought the original system worked off 28 volt aircraft
batteries. And of course the aircraft 28 v engine generators.
Batteries have a low impedance and might have done much to 'shunt'
brush noise of the various dynamotors? Could you try adding more shunt
filter to your 28 volt supply just to see if it could reduce the hash;
presumably coming back from the input brushes of the dynamotor? Or if
from brushes on the output side check hash filter caps there?
Finally could you unplug the dynamotor and have a cable from external
PS plug into that socket? No other changes whatsoever! Retain
dynamotor (perhaps attached to PS?) for future re-installation.
Good luck with your project.