Nice boat, bad motor

I am getting out of airplanes, and trying to go fishing! Problem is, it seems like boats aren't any cheaper. The '79 V-20 I just bought lasted about 1 1/2 runs, then the engine ( Mercruiser 470) failed. It will not idle or accelerate. Peaks out at 2800-3000 RPM, then decelerates and dies. After playing with it for several days, #3 cylinder lost all compression (cause unknow at this point) I am told that parts for this engine are difficult to get, so rather than fighing it, I am thinking of putting in a new I/O unit. Any comments on the parts situation, and mayby someone in the Biloxi, MS area who knows how to work on these antiques?