Trolling motor size

My son just bought a 2001 Hells Bay Waterman, Mercury 40, with deck. It is all Kevlar and no fluff.

It has factory trolling motor which is long dead. Think it probably was 55#?

Question… Boat weighs approx 440#, not sure on motor. He weighs 220.

Will be used in river and harbor.

What size trolling motor is needed?? 55# work ? or is it marginal…with our currents…