The first thing I would investigate is the linear drive unit. Basically you have a 12V DC motor
driving a push rod through a gear
reduction and ball screw and a 12V DC clutch.
Starting in the middle drive position (don't want to drive against any hard mechanical stops), apply 12 V through a 10A fused supply. The motor
should run in both directions just reversing the polarity of the motor voltage. If this works OK try applying the 12 V clutch signal through a 3 A fuse and see if the rudder moves again in both directions using the motor control voltage. If this works out OK, things look hopefully for a significant part of the system.
You can reconnect the AP power unit and see if anything works. If not probably need to consider new electronics
plays a little loose with the truth on needing a big ($$$) power drive units. Depending on the rudder forces and if any oceans are being crossed might get by with one of the older smart pilots off ebay (S1G, or X-5). These are typically capable of 6 amps continuous and 10 amps peak current
. X-5 will need a clutch signal (12 volts and a toggle switch).
I have been running a X-5 driving an old Benmar rotary actuator for years (mechanical clutch). Peak currents are less than 3-4 amps but an easily steered yacht through an efficient rack and pinion drive system.
Or you could just get out the old check book and spring for one of the newer systems from vendor of your choice (always good luck with Raymarine / Autohelm).
Might want to rig up a temp drive setup (clutch toggle switch and center off DPDT for motor drive ) and develop a little confidence in the drive unit underway before diving
into new electronics.