I will just state the obvious, the horsepower was literally cut in half. 15 gallons of gas plus 200lbs for the kicker and a empty bow, that boat is not going to plane. You will need to drop pitch on the prop.
Being a Johnson I am assuming they sold you a used motor.
^^^^^this.
Boats are designed to perform when powered within about 75% and up of rated max. They'll never perform at 50% of rated power. Best you can do is play with balance. 4 strokes are heavier and you have a kicker adding weight to the stern.
The fitter should've warned you of this. Sometimes owners are fine with being underpowered, but typically not.
Edit: I see the 16' Arima is often spec'd for 40-60 hp with some 90hp fitments yet best I can determine rated for 100 hp. So you should perform not terribly with 60. 125hp would be overpowered. Personally, I wouldn't go less than about 75hp with a 100 hp rating. Arimas are cool little boats being quite seaworthy for such a light craft.
The idea of getting on a dedicated Arima forum would be on the top of my list.
Sidebar: OMC went out of business in 2000 and Bombadier dropped the Johnson line in 2007 in favor of the Evinrude name (which they dropped in 2020). OMC tried outsourcing, lost quality control and built some turds. The industry lost confidence in what had been confident mfg. Never recovered. When Bombadier purchased, they assumed no liability for OMC mfg'd motors.
