Old outboards don't have the compression that a new one does.
Wear and tear over the years loosens things up and after a while,
you don't get as much push out of the engine as when it was new.
Outboards get tired from working, just like we do.
If an engine has ever been hot enough to lock up, the rings should be replaced. A common event is not enough cooling water and the engine freezes, but after cooling it will seem okay. If you check compression it will be less than before and produce less horsepower. If only the rings need replacement (small scuffs can be honed out of the walls) it is not a difficult job). The connecting rod caps and rods are not interchangeable and must be kept sorted out. The rings lose their tension when too hot and must be replaced. All modern engines are HP rated at the prop shaft. (previously at the crankshaft) I can't remember the year it changed, but likely Brett will know or find out. I had an OMC school near where I used to teach. They had a dynometer there for outboards. The master tech there would put a big outboard on the dyno and often get as much as 40 additional HP out of a 225 HP engine just through adjustments. A tune up kit for your engine may be all you need. Check the compression before anything else.