If it truely has 13.2 to1 thats even too high for 100LL blue AV gas in my opinion. That is the most common type of AV gas you're likely to find. What heads do you have on it? If you have 64 cc chambers you could drop it some by swapping to 76 cc chamber heads but still likey to be on the high side for 91 octane would be my guess without doing the math. There just is no way to do what you want without swapping parts.
You need to know exactly what you have to make a wise plan of action.