I'd go with Por 15 for inside the frame. The stuff coats and coverts rust, and a little goes a long way. It is a very thin paint that should take care of any rust issues you have, then maybe go with less of the "Tim the tool man Taylor" approach, as Don said, more power on one of those can be scary!
Had a friend put a huge marine patrol out board on a small cubby cabin fishing boat, used to be like putting your life in your hands going out in his boat, especially before he put a floor in over the exposed stringers. Lightening quick power and no stable platform equals disaster!