Not too many are still with us. Of the eight billboarded actors on the original series, four are deceased (Gordon Jump in 2003, Frank Bonner in '21, Howard Hesseman in '22 and Loni Anderson last year).
Of the others, Gary Sandy is 80, Tim Reid is 81, Richard Sanders is 85, and the only one still in her 70s is Jan Smithers, who's 76. Yes, I guess they could do interviews or promos for the station, but how likely is it that any of them would hop a plane to do publicity for a little group of Ohio stations, even if Randy treated them like royalty for the week? Also, would they be themselves, or in character? Time has a nasty effect on people's vocal range and quality. Keeping in mind that this is radio and that's all a listener is going to hear, how many of them still sound like the characters they portrayed half a century ago?
It's better to use clips from the original show, when these folks were (a) alive and (b) still sounded like the people they were back then. Leave the rest to the memories of viewers listeners.
(Edit: fixed my typo)