I haven't done an e-conversion on the Neptune, but I did one on a similar sized model of the Seawind.
I put the ESC up on the pylon with the motor and ran the two power leads through the pylon to the fuselage. I think you may be able to run the power leads along the outside of the Neptune pylon and into the fuselage. You will probably need to cut a hatch in the front of the fuselage for the batteries to be able to get the CG in the right place. Hope this helps.