The couple of hydraulic steering systems I have seen, have consisted of a (toothed wheel) pump on the  steering wheel and a double action hydraulic cylinder acting on a little tiller, above or below deck. If the maker of the system has been kind enough to fit that tiller with several holes, you can simply attach the cylinder to a hole closer to the shaft,  and thus making the tiller shorter and the gearing higher.

If that cannot be done, you will need a hydraulic pump at the wheel which pumps more oil per revolution than the one you have (or replace the hydraulic cylinder with one with smaller bore). That should gear up the steering.

Arne

 PS: Have you considered fitting a smaller steering wheel?