100 pounds to me, means a very heavy boat. Disregarding that I would recommend, a Feigao 10L, 80A ESC, on 8 cells.
Place everything so that the batteries will allow you any adjustment for CofG. Your CofG should be around 27-33% from the rear of the transom as a starting point. Move your batteries to get with in this range. Usually these offshore eBay boats are built in electric and nitro versions. You can check out the electric version as far as placement goes.
Oh, and just because one can not get an engine to run does not mean it's crap.

I've had several nitro's that run more consistant and last longer in terms of years then electrics.
Ryan