Qames/quake from 9front =). It can run LibreQuake with Malice as a MODs, and that's it. Quake, Quakeworld and everything for vanilla, no modern changes like QuakeSpasm or worse, DarkPlaces. If someone backported HL2 to the original Quake with reduced physics and still run under a Pentium III fast enough, it would be something astonishing.

I see impressive stuff with reimplementations such as Surreal Engine, but they will require far more powerful machines.

If Surreal had a software renderer (not requiring AVX or similar) running under an SSE2 machine, that would yield even more respect, because if your reimplemented engine runs in legacy machines the portability would explode. Just have a look on Scummvm on how many platforms and OSes can it run. Or the Super Mario port for PC, where some fork supports even 3DFX under DOS, and GL 1.2. Thus runnable under TinyGL with no 3D accelerators and even under Plan9/9front with custom tweaks.