Looks more like a Mercury built Mariner than a Yamaha-built Mariner. The Yamaha-built motors had a serial number sort of like 689 (or S) XXXXXX, where Merc built motors just had a number, sometimes with a letter thrown in. I also remember the carb being more centered; but it's been a while since I messed with one-which was mostly a C25 or C30 yamaha. The C25 and C30 are the same block, actually most of the motor is the same and a C25 can be converted to a C30-I just don't remember how.
If it's a merc engine, do a compression test first thing. They are known to delaminate the chrome lining from the cylinder walls, and as mentioned, there's no fix other than block replacement. I "think" there was a company who can re-line them but I'm not sure on the cost as to whether it'd be worth it or not. Worth looking into only if you have to. Hopefully not. Google search "mercasil block"-or something similar. Mid 1990's is when I got out of the Mercury business, thank goodness. Brunswick didn't like to pay warranty claims, or wasn't at the time, and for that and other reasons, we let them have their garbage back. We were a Yamaha powersports dealer, so it just made sense to start carrying their outboards, and we sold thousands of the 25hp 2 stroke/twin carb models. Every duck hunter in the area had one. They weren't as fast as the 3 cylinder model, but they were LIGHT, smooth running, and reliable enough even for duck hunting in flooded timber. The "mercury" guys (includes mariner) liked the Merc/Mariner for one reason, and one reason only. Shift in the throttle grip. I personally hate it.
For what you have in it, it'd be worth messing with as long as the compression is 90 psi or greater.