new engine

i have a 1989 v20 steplift with a new 225 h/p jasper i/o that i am not very happy with because it does not preform as well as my origanal merc 165 h/p i/o i used to get about 42 m.p.h. with the old engine but now with just 2 people i can get max 31 m.p.h the old one through a rod and cracked the block one day or else i would have kept it. i did not choose this engine my mechanic said was a great engine and to trust him. It is a reliable engine but not a powerful engine. is somthing wrong with the engine or is there a way to get more power out of the engine.

p.s. this is my first time posting