You can try using the plugin:
Available for free on the Hex-Rays website, there is also an implementation that uses yara
I have tested both and they do the job very well, with them it is possible to find possible encryption key in addition to the most common cryptographic patterns used by developers, assuming that the key may be in ...
As you said, the Lua engine is embedded directly into the game. It's inside plugins/GameManagerVC.dll (or GameManagerVC64.dll in case of 64-bit).
So, first step will be finding out, which version of Lua it is. So put this DLL into Ghidra or IDA Pro. The most easiest way of figuring out, what version of Lua it uses, is by searching for "Lua" string ...