Rejuvenating older cassette deck

As much as I have tried to "upgrade" my parents to the CD format, I have failed. So for xmas I bought them an older Onkyo cassette deck that can be controlled by the Onkyo receiver. Although the eBay seller insists that it still sounds good, what should I do to help bring out its best sound other than just cleaning the heads? Is demagnetizing (degaussing?) necessary?

Thanks for any help.