you need a 6 inch pipe (15cm) to get 50 liters per second at a head loss of 4.1 meters per 100 meters of pipe.
do you know anyone with a 3D printer?
you could 3d print various kaplan turbine blades, find one that works and then have it cast in bronze or aluminum. basically bury the PLA in plaster of paris, then burn out the plastic in a kiln, pour aluminum in.
automobile water pumps are sufficiently cheap you could harvest one for the pump seal. another option bend the pipe after the turbine, put a bushing on the leading edge of the turbine blade(and perhaps after it) and run the shaft out through the side of the pipe after the bend. you'll need a bushing where the shaft goes through the side of the bent pipe. but this will keep the generator out of the water, and if you can deal with water spraying out the bushing, you don't need a pump seal, which makes experimentation much easier.
another option is use a water lubricated well pump as your generator and directly install the kaplan turbine blade on the end of the motor. the motor will already have the water lubricated thrust bearing in it, so you don't need to add anything provided the motor is after the turbine blades. you can leave it as an induction motor if you're willing to spend a lot of time getting it running right, or add magnets to the rotor..
problem being the rpm of a 6 inch diameter blade in 40L/s water is probably too slow for the hp you're going to have available, in a 4 inch (100mm) diameter pump motor.
so you theoretically have 4KW of power available.
you could design the turbine to drive an 8 pole induction motor directly, but those motors are hard to find. if you have 50hz native power then a 6 pole motor might be a good option too.