Big move to change out to such a reduced HP engine. I looked at a boat (a Helmsman 38) where the original owner had ordered it with a 69 HP engine. We sea trialled it on Lake Washington and it moved the boat at or close to "hull speed" with no wind and flat calm conditions. I had no real way to know if that engine (HP) would be enough when facing 4-6 foot waves generated by 25 knot head winds. Also, we transit many narrows (rapids) in this area where you can face strong currents. For these reasons we went with a boat with the engine size that most builders provide. More horsepower and complications than we need, but....
If we could have found a boat (our size) within our price range with about 120 HP (as KK 39's come with) that would have made me more comfortable with it's reserve power.
Just some random thoughts with nothing scientific to back them up.
Welcome again, and enjoy.