Low compression in cylinder #3 of my 1974, 75 hp Chrysler.

I own a 1974, 75 hp Chrysler that I've had since 1976 mounted on an 18ft Starcraft 18 SS. This engine has served me well over many decades with no major repairs. However last year I was out on the lake and lost power while underway. Not all but at least half. When my mechanic went through the engine he discovered that cylinders #'s 1 and 2 had a compression of 150 and cylinder #3 was 75. Any ideas what the issue could be? He says that the engine is old and parts are difficult to get and the repair would probably cost much more than the motor is worth. But I love the power of this engine and I haven't found a 75 horse that can do what this engine could. Any help or suggestions?

You could have had a plugged carburetor that went lean or an overheat situation. Most of the parts required to rebuild it are available. The pistons are available up to a .040 oversize and the gasket set is on ebay. As long as the crank and bearings are good should be doable. As far as cost, the mechanic is right it will cost more than the motor is worth, if you do the labor yourself you can save a lot of money and learn something. Or a good choice to replace your motor would be an Evinrude/Johnson 70 hp motor 1978 or newer will have all the power of your 75 chrysler. Just some options for you.

I worked for a Chrysler Dealer back in those days and really liked the motor. I'd probably fix it.

Looks like he's not been back since he posted.
But fixing it yourself is the way.
Maybe just check the head gasket?
I know mine go bad every 4-5 years.
Salt water is a B