Good luck my friend!
Used engines in the year and horsepower range you are looking for are few and far between. Especially if looking for a 4-stroke.
Most you will find for sale used are off Tournament boats with a ton of hours on them. So, you could end up buying someone else's future problems.
I would consider having a reputable mechanic rebuild what you already have if possible. The older engines burn more fuel, make more noise and smoke more. But, unless you can buy new, I think you will be money ahead to rebuild and pay the difference at the pump.