As I said, I was looking over some old C code from my DOS/Windows time about 25 years ago. I finally found most of what I needed so the main .h include file will compile without error. It was not as bad as I had thought. The biggest thing that was confusing me was the plethora of include files and architecture variations I did not have to deal with back then. Trying to find where something is typedef'd today is a lot more complex.
Nevertheless, if I continue this project I may be asking for some assistance from time to time.
Basically, I wrote a scuba diving "log" program called "Dive-In" back around 1988 for the same reasons Linus Torvalds did in the last few years. Mine was better than SubSurface in a several ways and SubSurface is better than Dive-In in a number of ways. I still use Dive-In and have around 780 dives in it, but I have to run it in DosBox or WINE and that causes some issues for me. I'd like to export the data to a format that SubSurface can import and that is essentially the project.