You can make one of a sort. Find a rather large word in the script, all lowercase, calculate the numerical difference between the letters, write a program that searches for that word with each letter offset by the same amount, and you'll be able to build the letter key using that. Punctuation is harder. If there was a way to unpack, edit, then repack the game, this might be easier.
ALTERNATELY, WE CAN USE THE LETTER KEY I GOT FROM RIVIERA GBA.
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