My favorite classic NPP simulator is SIMULA-C by Ralph Reuhl. I'm only finding this report [1] (from my comment history, hah), but both the C source code as well as MS-DOS executables were available. Back when I knew what the URL is, the Wayback Machine could fetch the .exe files but not the source archive.
Anyone remember this one? A screenshot can be found in the PDF at the end.
[1] https://inis.iaea.org/search/search.aspx?orig_q=RN:29043408