So many variables.
Prop size and pitch.
Boat load and weight distribution.
Weather conditions.
Speed or rpm.
If the engine reaches its max rpm with the intended load then the prop is correct and the engine will be more effiant through the entire rpm range. If the engine can cruise at around 3500-4000rpm at a speed you are happy with then fuel consumption is going to be better.
Is not just the fuel consumption you have to consider on an old two stroke like that but the cost of spares that must be getting harder to come across these days.