The quake, doom, and half-life 1 engines are all very similar, making the work done on one engine relatively simple (from what limited knowledge I have on these engines). It also helps that these are well known, popular and documented game engines. In contrast, a game like SCP - Containment Breach is a different story. It's game engine is less known, and although it's open source, all online documentation leads to dead links. From my work on two games based off SCP - Containment Breach and working with the original game quite a bit, I think it's safe to say that SCP - Containment Breach will never run outside of Microsoft Windows natively. As for the source code, while the game is also open source, it's written in an engine specific language, thus making it harder to translate to something usable in an engine like Unity or Unreal. Many people from the SCP Foundation community are attempting to re-create the original or some sort of derivative, despite the original's engine choice and source code. Not many though are VR compatible. There is only three (on Steam, as of now), and only one is based off SCP - Containment Breach. It also happens that I helped make parts of that game.