I had two 1500w 5.7 lamb motors in my old ninja and a uprated 150psi shurflo pump and no issues with the tank imploding or the machine tripping any electrics.... you will struggle to fit and 6.6 motors in there and then there is the added cost as you may as well put the money to a newer machine. there is also the heat issue with 6.6 motors in such a confined space.
If you go above a 150psi pump on the older ninja you will need to upgrade the solution internal hoses too as they i believe are only rated to 200psi..
But two 1500w 5.7 motors will go in with no problems..