• Modders are working on an offline mod for troubled game The Day Before.
• Development studio Fntastic has closed its doors and the game’s servers will shut down in January.
• Luci0 and fskartd are working on a crack that would allow players to play the game offline.
I think that’s how the open source ports of Doom and Quake work. The shareware files are enough to get it up and running, since they’re free to distribute.
As for the engine in this specific case, it’s Unreal, possibly without any modification, so the engine is already available with source.