Can you give some sample XOR keys to check for, and what files they are for?
scenario/1 プロローグ.ks - encrypted with A0.
scenario/2 スタート(霊).ks - encrypted with 80.
image/alice01.bmp - encrypted with 20.
image/alice01_me.bmp - encrypted with D0.
sound/ali_0000.ogg - encrypted with 50.
sound/ali_0001.ogg - encrypted with 20.
bgm/bgm01.ogg - encrypted with D8.
bgm/bgm02.ogg - encrypted with 60.
Here, hope you can do something with it.
UPDATE: I changed 1 byte in scenario/1 プロローグ.ks and tried to bruteforce the correct key after that. I actually did manage to do that (the correct key turned out to be 80). Looks like ks-files are encrypted with 4-bit XOR.
UPDATE2: I can now tell that it's perfectly possible to replace japanese text within the files with other japanese text (you'd have to bruteforce the XOR key though). But if I put english text, the game hangs. Any ideas?