The boat is not under-powered. They originally came with an AD 21 producing the same horsepower. The difference is the max revs of the original engine was 2300-2400 rpm. The new engine should max out at 3000 rpm, so it seems you are not getting enough revs to develop full horsepower. It is likely you are overpropped with the current set up.
Original gearbox was 2.04 ratio and prop was 16 1/2" diameter x 12"pitch.
Here's the manual if you don't have a copy.
So - the original engine/gearbox/prop setup was all matched. If you still have the original gearbox and prop, and you have a different power curve on the new engine. The prop you now require will have less pitch (or "twist") which will allow the engine to get to a higher rpm to produce it full horsepower.
First - check your gearbox ratio. Then contact a good prop shop to see what they now suggest. As a rough guess maybe 16 x 10 now.