[...] because the game's engine is complete unknown.
This game was made using Clickteam Fusion. You can generate a project file (or extract assets) using this tool:
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No idea if you can recompile the game, but I can't see why not -- the project file contains everything.
Unfortunately, it looks like all the text is hardcoded -- no translation files or anything. If anyone plans to translate this game, going to the Event List Editor in Clickteam Fusion and searching for "Draw text" should get most of it.
I haven't checked, but I wouldn't be surprised if the dev's other games used the same engine.