My SR4 runs with a 30 HP 4 stroke Tohatsu, not underpowered i would say. Getting it on 27 knots 1 person (90 kg), and 24 knots with 2 persons. (+ 90 Kg)
With 2 persons and open flooding hull i wouldnīt like to go on nasty conditions, cause than itīs little underpowered. Alone (1 person) no problem.
A 4 blade Mercury spitfire prop 9.3 x 11 gives a much more quicker holeshot and acceleration than any 3 blade prop i figured out, even if hull is flooded with 1 person getting directly on to plane.
40 hp would be fine, 50 hp will make you a happy chine walk dancer!