How can I get back up into flying after not flying for over 20 years?
I do not have a sanctioned answer, but I can give you my take on an overview of what I would plan for someone in your situation.

Your certificate duplicate can be ordered.

You will need a through review of airspace (things have changed), weather gathering (also changed), AIM stuff (changed runway markings, phraseology, etc.).  I would urge FAAST courses for various topics you might feel you can get brushed up in.

On the air side, if we were flying, we would do a "checkout" in the 182, covering the systems, cowl flap operation, targeted power settings, trim usage, etc.  A little airwork, steep turns, turning to headings, power on and off stall recoveries, emergency procedures and landings, takeoffs and go arounds.

Everyone is different, but the last guy, similar to you, flew C208 and stopped in 2003, and did not fly since then.  This summer he hit the books, watched a few videos, took a practice commercial written, and flew 3.4 hours with me.  I was real comfortable signing him off, and his wife, who was the concerned party, was real happy with the results.

Two months later we did a instrument proficiency ride, and that was a little more difficult for him, but he completed the ride in two flights and less than 4 hours.

Good luck and enjoy. Have your instructor put a new high performance endorsement in your new logbook for the 182.