50:1 fuel/oil mix
The overboard indicator hose outlet is located on the starboard side of the lower motor pan. There is a little plastic retainer/restrictor that is located in the end of the hose, debris loves to get caught in it...There should be a fairly strong but narrow stream of water shooting out of this tube. If not, try blowing back through the restrictor, or removing it from the hose and cleaning it out....Usually this is all that is wrong here....
Yes, there will be a fine spray of water/exhaust coming from the two exhaust relief holes as well....
This engine does use a shear pin, later models used the splined shaft like on your 15hp. This is an old gearcase design that has been around for many years.
Electric start on these engines was fairly common, so keep searching for the bracket, you will find it...I'm pretty sure the same bracket was used on the 25-35hp models from 1977 on up through the 90s, but don't hold me to this statement...Check actual parts lists/numbers to be sure.
The correct spark plug is the Champion L77JC4. The QL77JC4 can be used as well. The original number was L77J4 from Champion...Gapped at:
Looks like your engine was manufactured in Canada. These are fairly rugged engines, but don't idle as nicely as models from the 50-60s. The CD ignition is fairly simple and easy to understand/repair. The cooling system can be difficult to service, and watch out for worn clutch dog/gear lugs, replacement parts are very expensive.