jerni21,
Not being 'into' engines I can't really tell you anything about them, so other suggestions would mean more than mine would.
As far as access to the engine, or the pull-rope, just make the superstructure removable (think it's already that way).
The thing about displacement hulls is that they aren't made for speed to start with, at least not like others. They 'push' their way through the water, not on top of it. That means that while you can make them go faster, it requires a huge amount of power increase. Gotta overcome all that ballast too, get the thing up on a plane... forget it! Might think about gearing the engine down so that you have lots of 'torque', as opposed to speed. Then you could pull that 14 foot jon-boat...
- 'Doc