Are you sure it's a freehub/cassette system, or is it possibly a 5s uniglide freewheel mounted on a threaded hub?
Given the generation, I suspect it might be the latter, in which case you're in luck.
Sticky freewheels almost bounce back of they're removed, soaked in solvent (kerosene, mineral spirits, turpentine, etc.) dried than have a bit of oil dribbled in though the gap between the inner and outer bodies.
If you're not sure, one hint is the shape of the hub between the flanges. Freehubs have a bulge inside of the right flange, whereas spin-on hubs are symmetrical between the flanges.
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