I may not be much help, but, you have no others help, so...............
I doubt this as easy to fix (parts and their cost) as something newer, but, if I had one, I would tinker with it and use it if it would run 1/2 decent and not blow my boat up.
This link may be good, you going to need to browse around;
Old Marine Engine
Notice this link at the same web-site (You may learn more, asking here);
Old Marine Engine: Gray Motor Co - early 1 and 2 cylinder
Scroll to bottom, you may have a GM-71
Gray Marine Engine Manuals, Grey Marine
The manual may be worth more than the engine. If you have a good mechanical knowledge, I feel you should be able to work without a manual. Though torque specs are nice.
This next article tells of what someone do to rebuild one;
Gray Motor Company Antique Marine Gas Engine