http://www.gamecycle.co.uk/index.php?main_page=product_info&products_id=16Looking at the advertising blurb, I suspect that fitting more powerful motors would be a waste of time, partly because the hull wouldn't handle it, partly because the "custom" control would die rapidly. Again looking at the advert, it was designed down to a price, not up to a quality.
Normally the motor shaft is connected to the prop shaft by a universal joint or rubber coupler with end bushes, often secured with grub screws. In this type of boat its often a force fit that was never intended for disassembly without destruction.
Best to enjoy it for what it is, then figure out what you would like for a second project and build one with hobby grade, rather than toy grade components.