With a motor that only turns in one direction, options are a variable pitch prop, a gearbox, a "Kitchen Rudder" (look it up via google) or use the motor to drive a generator and move the boat with an electric motor and an associated ESC.
Having reverse is not an optional ornamental extra with a large model boat. Reverse is your brakes.
If the engine drives a 3 phase alternator, that can, in turn, drive a 3 phase motor (or brushess, as modelers know them). Speed control of the motor by the speed of the engine, reverse by operating a relay reversing the polarity of any 2 of the 3 wires.