I have this poor old I believe the year is a '89 Dura Ace RD (RD-7401) with a stretched spring in the cage. Is it possible to respring it or would it be more simple to find or buy a new spring for it? here is a picture.

seems like it simply needs to be put back together correctly.

the torsional spring that keeps the derailleur cage in position, seems to be either broken or not wound.

from what I know is that the cable angle adjuster was screwed all the way in which must have messed up the angle of the cage, how hard is it to rewound the spring?

it doesnt look like thats the issue
the derailleur cage is not in the proper position

simply take the pulleys off, lay them out in the order that you're taking them apart

when you get the top pulley off, you will see a screw or allen in the cage, that bolts to the parallelogram of the derailleur
unscrew
and you will find a torsional spring

take it a part and play with it. you can't really screw it up.
don't unsnap the big return spring inside the parallelogram, its a pain to get it back into position

look at this pdf
https://techdocs.shimano.com/media/te...9830611841.pdf
it will give you an idea of what your derailleur is like

from your picture, I think that part #8 (on the pdf) is either broken, or its simply not wound correctly.