The oldest Chess engine you can find for free online is cray blitz https://craftychess.com/downloads/crayblitz/ which was the world computer chess champion in 1983. Unfortunately A: it is not UCI compatible and B: the oldest version available is the 1989 version. I asked Harry Lewis Nelson himself yesterday and he said that he doesn't have the source code for the 1983 version anymore. Unfortunately any farther back and the original author doesn't appear to be alive anymore. (harry himself is 89)
The oldest chess engine that can be called a champion is kaissa https://www.chessprogramming.org/Kaissa however the best I can do is give you a rewrite in turbo C http://greko.su/index_en.html I figure 1977 is as old as you're going to get. Hippke can probably do most of the heavy lifting from here.
I agree with the use of stockfish 11 as it appears to be the best engine with poor hardware, NNUE requires more RAM than actually exists on a 1997 machine.