Some ideas:
This hydraulic motor may be useful, though I suspect the shape of its performance curve may not be like that of a turbine. You MAY be able to play with the efficiency tables to get close.
You may also try to piece two rotational converters together like in this block: https://www.mathworks.com/help/hydro/ref/doubleactingrotaryactuatoril.html (note the first picture).
In fact, I believe in Simscape Onramp, there is an example of "Hydroelectric Power Model", which uses this converter block to get mechanical power from hydraulic power. You may attach variable resistance or restriction blocks parallel to the "turbine" to model the leakage. Parametrization will take time, and you'd be likely to need some 1D or 2D lookup tables to mimic the turbine curves/surfaces. The 2nd picture in the link above shows the basic idea.
