why replace what is not broken?

Minecraft.

There used to be a rudimentary port of it to barely run on old raspberry pi’s. Modern Raspberry Pi’s are good enough time run the actual game (albeit barely). It’s also frequently hacked with, cloned, and demaked, and is the most popular game on the planet.

Heretic.

It has to be something that fits in a relatively small package so it can fit in the space constraints of old hardware. Anything with true 3D levels and texture mapped models is right out.

It also can't be predominantly written in assembly. Making assembly code run on a different architecture is practically a complete rewrite, not a port.

This leaves a very narrow window of games written right around the time of Doom.

I think it should be a game that barely runs on modern hardware entirely because it's poorly optimized. It would be funny as hell to have a game like that running on toasters if done by talented devs.

I thought of Crysis also when I read the title. But first, I'm down for Quake and more specifically QuakeTF (team fortress). I spent way too much of my youth playing quake and qtf (honorable mention to UTF).

What about one of the old C&C games that just got open sourced by EA?

Cool Spot

Doom... Eternal.

Duke3d might be a next logical step in terms of probability of running on all kinds of random crap without being too big or resource intensive.

Xonotic

Skyrim