The .91 could probably pull your Cub and another plane off the water. Seriously, you won't lack for power. And the Golden Knight is identical to the standard except for the finish. Just curious, are you going to mount the engine sideways or inverted? That engine is tall enough that it will stick out pretty far. I fly 2 planes with Saito's, a .56 and a .65 and mounted them both inverted, more for aesthetics than any other reason, and neither one has ever missed a beat. My .65 actually is in a 1/5 scale Cub that weighs over 9 pounds, and it will take off with little more than 1/2 throttle and cruise at 1/4, so I'm sure your GP Cub could do very well with a .72 or an .80, even with the higher drag and weight of floats.