You might have answered your own question about the low compression numbers. Try turning the motor over with the drill set at high speed and see if the numbers improve.
Hard starting/poor running is another can of worms. These motors like to start on the rich side so opening the needle valves a quarter turn or so more from their optimum setting will help them start especially if it is cold outside. I’m sure all major ignition parts have been replaced along with new plugs. NGK B6S are good plugs for these motors.
About the compression test on the forty horse, was the motor being roped over or spun with the electric starter? As stated, those numbers should be well over 100 PSI.
When checking air pressure gauges, I like to compare them to a known good one which has been calibrated by an instrument technician. I find the cheapo smaller gauges don’t read accurately in the upper or lower pressure ranges so I set them for around 80 PSI which is approximately what these and other trolling motors should read. My favorite tester to use is my digital one, which is accurate at any pressure range.
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