Is your transom in good shape, a newer / faster motor will be heaveir and drive your CVX faster then it was designed for ...
How fast is your CVX now ?
Merc makes 115 /125 HP 4cyls. and 135 / 150 HP V6cyls. 2 strokes also.
I'm not a 4 stroke fan, heavy and expensive ...
Suggestion # 1
Rebuild your Evinrude 120 / cheapest route, with some setup / tuning should be good for mid to high 50's.
You could add a few go fast parts and make a nice fast boat ...
#2 ...
Evinrude E-tec 135 HO / Little faster, expensive, would still be with in boats HP rating.
#3 ...
Evinrude 150 HO / Very fast, very expensive. would be good for mid to high 60's.