I'd say use old connies, they're lubed so even better (no stiction to the rod haha). And they're even cheaper than used inner tubes! A coloured neoprene shock sock around to top it off and protect it from uv-light. Seriously there are so many solutions. But since it is as cheap (or even cheaper) than a used air-oil shock (compare that to a 250€ shock service), it is way easier to just buy a new perfectly fitting elastomer from the multiple available sources. I was planning on trying with a new monroe car bump stop as they seem to fit the bill. Just like old MCU's exposed to UV light, they fall apart with age (see pic). They cost like 15€ new. Not sure if it's worth the hassle, but good for fun. I think the used inner tube is excellent for replacing that lower rubber dust cap, as those usually also disappear with age and previous owners on proflex bikes. But seriously, what's up with fitting road tyres to old mtb's?? Makes them look even more dated (wheels look even smaller) and takes away their original beauty.