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I wouldn't panic yet. I would do a leak down test if possible. A borescope to look at the piston would help. If that is not possible get a led flexible neck flashlight. Lowes has them for about $10. Look in the cylinder and look at the edges of the piston. If the piston is OK looking, not erroded no obvious damage, then I would get a can of Marcury's Power Tune. It is a spray carbon cleaner. Follow label directions and treat all the cylinders. Put a bit more in the bad cylinder.
Do the treatment twice and then recheck the compression figures. If the number comes up then the problem is sticking rings. Then treat the fuel with Mercury's Quickleen in the fuel.
If the piston is distorted or any signs of rubbing on the cylinder walls then the engine will need to come apart.
Post back what you find.