As John posted, you basically just need to obtain the new plastic certificate. With that, you're totally legal.
Next step would be to go get some refresher ground school, and some refresher dual instruction. The airspace and other environment has changed over the past 20 years, and taking a ground school would probably be the best way to gain understanding.
The dual instruction really depends on how much you retained the "feel" for flying. Maybe you'll pick up the knack again and just need a couple of hour. Or the instructor may feel you're a bit TOO rusty, and want to brush up on the skills a bit more.
Since you've already got a ticket, though, it's not like there's a target length for the syllabus. Don't go solo until the instructor...and you...think you're ready.
I've been in a similar situation...though not as extreme as yours. I quit flying when I went into the Air Force (actually, I flew satellites, but it ain't the same) and didn't re-start for about eight years. About an hour and a half of dual got me qualified and comfortable to fly the '65 C-150 I'd bought. Another hour of dual in a 7ACA Champ, and I was signed off to fly taildraggers again.