The quick and easy answer is YES, replace the batteries. If they are that old I will bet just about anything that their dead. Batteries are too cheap to risk a plane on them.

But one thing that does concern me is the age of the radio. If it's 20 years old there is a good chance that it's illegal to use now. All radios these days have to be Narrow Band. I think this went into effect in the mid-90's, and back then it was called "Gold Sticker" because all radios that complied with the law had a gold sticker on them so show they were narrow band. If your radio is 20 years old there's a pretty good chance that it's not narrow band and is going to be illegal to use here in the US. Of course the radio still works, but you won't be able to fly at any AMA fields with that radio.

Something to think about

Ken