I have one of these, for $630
They use a slightly larger "trolling motor" propeller, (came with 2 props) so a larger trolling motor propeller can act as a replacement.
The motor is far stronger than the push power of the propeller, so I'm thinking of manufacturing a larger than stock propeller.
These are all "low speed but high torque". They will never go faster than 6 MPH no matter how small a boat they are on. But they will spin at the maximum rated 5,500 RPMs (or whatever it is) the second the throttle is pushed, even if mounted on a sea wall, and they will push absolutely anything, slowly.
The shaft length is crucial! It don't matter how many horses you have, if the prop comes to surface and over-spins in rough conditions. (Shaft length on that linked product is about 23 inches)