You know...if the fidelity of all of the other tracks is similar to this one, I don't think you're going to make much progress. This goes along the lines of the old saying, junk in/junk out.
No offense to them, but it kind of sounds like the microphone they recorded with fell down behind a speaker column and they didn't notice until after the session.
No telling what kind of recorder they were using, when they did the original. The record heads could have been dirty and the azmuth could have been way out of whack. I like the info you have already been given to adjust the azmuth of the tape heads to match the tape. That will get you the best high frequencies possible.
BEFORE you adjust the azmuth of your tape heads to match this tape, be sure you have a way to put them back where they were.
And make sure you degauss the heads on your deck, to keep from adding more hiss than is already there. Doing these things might bring more fidelity out of the tracks, if you're lucky. Good luck!