You won't be able to match the performances of electric with a Cox .049 because of prop restrictions and frictions in the drive train. On the other hand an air boat design as used for tethered speed might be very fast but tricky to implement as r/c. There is some good ideas in that site about small electric (a .049 Cox has about the same power as a Graupner Speed-400/6V)which can be converted to small IC power:
http://www.microhydros.com/