I have one. stock motor with and macs 3.5 pipe cut to 8 1/2 inches and an x440 two blade. opened up the vents on the cowl and drilled some holes in in the back of the cowl to let the air out and put one of the brushless rudders on it had to open up the water pick up hole on it but i got 2 mph...
i use an Octura x440 that is thinned down and has some cup added to it you might also try the x438 both of these are two blades or the x437/3 or x440/3. i just got the x438 and x437/3 haven't had a chance to try them yet
if you want to keep it still rtr class legal i would use a O.S. 18 cvrmx it will last alot longer than the AQU 18 if you want to make it one fast cat i would put an O.S. 21 vz-m. thats what i did with my mean machine went right in no problem