You are what we call an “atypical applicant”, and review committees will not expect a typical application.
I don’t believe a recommendation from a casual professor three decades ago will prove valuable to a review committee, and think you should find a better way to tell the committee what they need to hear.
In terms of holes in your application package, I would think that what your more contemporaneous recommenders couldn’t provide that an old professor might is some statement that they believe you can successfully complete the coursework in their program. Nobody wants to accept students that are more likely than not to flunk. This is probably most important for PhD programs than Masters programs, as there’s a bigger financial and time commitment to the students, so flunking out is more tragic. This is less important for Masters programs, which happen quicker, with less financial support from a department. This is why Masters programs are easier to get into.
In any case, if your GPA was a solid B or higher, that might be enough to convince a committee. If it was lower, you should probably include something in your application package telling the committee why you don’t think their coursework requirements will be a problem to pass.