ok well first of all you will be droping about 500 bucks if not more to go brushless !

you well need to get a new drive line , cuz there is no way that that stock one will handel it !
and then you have to decide if you want a twin BL set up or a singal set up ?
and if the prop blades are counter rotating then a twin set up will be eazy , cuz there is no timing on a BL motor , so you can just switch the "black and white" wires ! , you will need to decide onthe motor , if you are going BL then you can run both motors off of 1 esc ,or 1 motor off 1 esc !
if you have 2 esc's then hook one 6 cell pack to each esc , simple !
but if you have one esc then hook up the 2, 6 cell packs in series so you can get the most volts !
also if you run 1 motor make it a larger BL motor so it can move the boat faster
have fun deciding !
run a flex shaft what ever you do !