Frank,
There were lots of changes over the years. Sponsons were added for more weapons and antenna mounts and the island went through additions.
I don't know of any aftermarket conversions, but I can suggest a few things.
Most importantly, do NOT use current photos of the ship as a reference for a 90s Nimitz. When the ship went through its refueling and complex overhaul in the late 90s, many changes were made. The mainmast, island, radar tower, etc., are very different today than when your son-in-law served.
I believe your best bet will be to use the Theodore Roosevelt, or the Abraham Lincoln, as your references for some simple scratch building. While antenna mountings, and other details will vary subtly, your model will be much closer to the Nimitz in your time frame using those ships.
Check the Navy News website at www.news.navy.mil for excellent downloadable photos.
Mike