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.
Yep! The German version contains a tutorial and a manual, that's why it's larger; unfortunately the Wayback Machine did not go back in time (since I last saw this page) to capture the source code from 2002. :)
I emailed the author to see if I could get the source code to update it for a modern platform. Given the amount of link rot since that time it is unlikely that he replies but hope is eternal.
I started the original in an emulator in the meantime, which reminded me that it had a quite slow simulation rate, which is also tied to responding to keyboard commands.
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