If you are trying to drive only the 60-90deg of rudder swing with a motor designed to spin, I think you will find the motor torque way too low to directly drive your shaft, unless you get a huge motor! You would likely need to gear
the motor output.
Anyway.. to size the motor: you need to know the volume of oil
pumps out lock to lock. then you need to match that to the motor. So, If you want to direct drive the rudder shaft, say 1/4 (90 deg) motor turn, and your helm
output is, say, 1L lock to lock, then you need a motor with a 4L displacement
per rev. Pressure is the next key parameter. You need to find the spec on the helm pump
and see what the rated output pressure is (or better, put a gauge in-line while loading the helm by hand and see what you get). The motor spec's will allow you to convert pressure to torque. You need to check that against the output force of your current
ram times the perpendicular distance of the ram to the rudder shaft and verify the motor torque is equal or greater.... good luck!