i would gto with twin 540 motors (you probably need to upgrade the esc too....try using the motors and esc setup like useed in the traxxxas villain.....does your boat use "vector steering"? if so, trash that....that steering system doesent work as well as the good ol fashoin rudder and strut combo..
go to
www.offshoreelectrics.com they have the motors and esc for the traxxas villan (youll probably need to get new batts too) and also carry some rudders you can use...
if you just looking to upgrade the motors and nothin else, sorry to say, but the stock esc (if its anything similar to the one i had in a different boat, but with the same setup) wont even handle an upgraded battery, let alone higher powered motors...
could you get some pics ofthe inside of the boat? and some of the transom...i still dont know if this setup uses vector steering, so that would help...