I know of KGoldrunner. That looks to be around 10,000 lines of C++. I have no idea how approachable the code base is, but it’s definitely smaller than I expected, which is promising.

(Its README at https://invent.kde.org/games/kgoldrunneror https://github.com/KDE/kgoldrunner is over a decade out of date, but it looks like the actual code is still being maintained so it compiles, though it may not be getting new development.)