Not sure what the exact situation is with the Black Mesa mod; I think it uses Half Life 2's engine, though.
\\NT Update//
Almost finished a set of patches to correct the following problems; they're based on my current files, so some new features might be sneaking in early.
Bug-fixes
-) Sorted out the eternal rain, and replaced sunset 'time of day' with 'overcast & raining' instead; also sorted a problem where the screen tint would be lost when opening the menu/finishing a battle and added handlers to temporarily stop raining when indoors.
-) Sorted out the potential loop that can occur when using the new save option during the Highway minigame.
-) Tackling a problem with Materia in the caves triggering their battles immediately rather than when being examined (pending)
-) Sorted out Aps' HP, and various AI glitches related to the now defunct difficulty modes up to the Mythril Mines (difficulty options were replaced with Original & Arrange modes; the changes made by Arrange are functioning to a limited extent within enemy AI, but consider this to be 'in development' for now).
Features
-) A new event plays in Kalm, where Yuffie will be recruited as a mandatory party member:
https://youtu.be/gbMC4e8AkCcThe PHS event is no longer present; this instead plays out when leaving Midgar for the first time and forming your first party; for players 'caught in the gap', several locations including Mythril Mines will discreetly unlock the PHS command (it still needs the world map/save point to enable). Players who have already completed Kalm, or who have yet to clear the Highway minigame, before applying the patch will not be affected.
-) Weather is now toggled when resting at an Inn in certain towns. I've opted for a lighter blue hue + rain over the rather garish 'sunset' hue. This'll be used for toggling NPC scripts in towns at a later date.
-) Midgar, Kalm, and the Chocobo Ranch have been re-specced. This is part of an ongoing playthrough I'm doing to tighten the mod up again; it's went through a lot of changes, some subtle others major, since the original balancing run was done so it's high time to get it done.
-) Did some minor work on the game's dialogue and text boxes. Consider this to be in development too.
I'll have the patches ready in a couple of hours or so.