As said, it's an aging problem, along with exposure to sun light, chemicals, etc. You might try posting a "Wanted" notice on here for re-conditioned "The Fiero Store" Steering Wheel. They did have them on the site for a while, but I just checked, and appears they don't do that anymore. You are going to have somewhat the same problem buying a used wheel, as you don't know what's it has been exposed to.
Ya, if you leave the original core and just recover it yourself, you dont solve the goo problem, It will still come out on the new cover. Good time to switch to aftermarket wheel unless you just want to keep it stock. I used Momos myself.
They do AMAZING work... But unfortunately they are a bit pricey for some Fiero folks. Something like $350~$400 to recover the wheel if I recall. I personally went with a 96 Firebird wheel. More modern looking, a tiny bit bigger around Easier to turn), and larger more padded wheel for a nicer feel IMHO.
[This message has been edited by JohnWPB (edited 08-31-2014).]