Hi gwharris,
I'm a Poseidon Odin fan myself, no experience with the Cyclon.....I love my Odins, but parts/service can add up if a reg needs a fair amount of work...the only way to answer your question is take it to a Poseidon dealer and get an estimate.....the good news the Cyclon has changed little, if any, over the years, so parts shouldn't be an issue. I agree with rescuediver009, assuming the unit hasn't been abused, and only needs the 'soft' parts replaced (o rings/diaphrams/etc.) , I could see you spending as much as $200 to get it dive-ready.
Another question only you can answer is how happy we're you with your 'old' Cyclon? If you haven't dove in 20 years, update your Cyclon, and then get to dive a more modern regulator, there's a chance you'll be upset that you spent money on the Cyclon, you might feel it breathes like crap in comparison to a new regulator.
The good news is for probably, at worst, you will have a functioning, at least adequate, divable regulator, even if it doesn't end up being as nice as a new one.....if you are fortunate (and since I've never owned a Cyclon it's hard for me to say) it might still have performance competitive with a new regulator......but that's a gamble.
Karl