You've got a classic! It's great to know they hold up that long. It's a major purchase that's hard to justify, but amortized over 20 years, the cost is negligible.
It might be worth it to seal the seams before treating the rest of the tent. I've had great results with Seam Grab on cheap ground tents. The pharmacy at my local CVS gave me a free syringe and I bought a cheap make up brush to apply with. Squeeze a bit of the Seam Grab into the syringe, run an 8" - 10" bead along the seam, and brush it in until the thread darkens, indicating permeation.
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