The higher the compression, the more pressure put on the piston, rod and crank. On the stock engine I would stick with the 8.5. I believe at 12.5 the engine would have a short life. One friend I talked to races lawn mowers, he has 12:1 compression running on methanol and has over 2500.00 invested just in the engine, special crank, rod, piston, reinforced aftermarket block with a billet forged head. Since he takes no short cuts he could not give me an answer as in your case. He said that these engines are low rpm, and hp and high torque engines designed for normal intended use
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