Hi - I've got one of those and once I've progressed with some of my other builds I'm planning on replacing the old MFA 850 with a brushless.
Being quite a heavy beast and wanting decent speed but not manic I'm looking at one of the Overlander 50 dia. brushless outrunners which will provide the torque required and use a 45 or 50mm 3 blade prop.
Definitely LiPos for me. The 7.3v lead acid was fine in the boat weightwise on the water but too heavy and unwieldy for moving around and launching and with LiPos you can easily up the volts without much increase in weight.