https://www.moddb.com/mods/half-life...mission-demo-1
Seems cool.
I could not finish the original Half-Life: Decay when I was playing it (stuck at a point which requires 2 players pushing 2 different valves together) and I gave up.
Very nice to see someone working on a better single-player version of the game.
Currently only a Demo version is available and can be downloaded from moddb link above.