I'm looking for that authentic 70s/80s movie sound of movies that haven't been preserved correctly / recorded correctly. In any case serious issues with pitch and all of that. Not sure how to explain it any better. Hopefully you know what I mean. I'm also happy with any example of a music track that is exactly what I mean. (I don't necessarily need to engineer it myself; any example of something of what I mean is good enough.)
I agree with Mark. A Tape Emulation plugin might be a good option. I can highly recommend Satin by U-He. it has great options to beautifully (or badly) degrade sound (besides enhancing it nicely). You can add some distortions (try a slight amount bit crusher) - lots of free plugins out there. simply google it for your desired platform and DAW.
If you want to effect material then Speakerphone is the most thorough tool I know of for the job:
Awesome, but expensive.
Wow and flutter are perhaps part of the sound you are getting at. If speakerphone is too expensive there are probably some cheaper plugins out there to simulate that, then some EQ might get you the rest of the way there.
Posting a youtube example of the sound you're getting at would help get specific answers.