From my Kohler manual:
"A difference in pressure reading between cylinders in excess of 15%, or readings below 90 psi can indicate the following: Blown head gasket, warped or loose head, worn piston rings, leaky valves, and excessive combustion deposits."
You're on the low end of the acceptable range. Did you ever pull the heads off and fix the valves? If so, you've eliminated everything but worn rings. Does the engine smoke or use excessive oil (more than a quart in 25 hours)?
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