A few years ago during winter storage ashore heavy rain dislodged a rock from the slope above. It hit our rudder causing a great deal of damage to the rudder and steering, the rock weighed several tons....
So, the rudder required to be repaired and from your photo the construction looks exactly the same..
The rudder is an aluminium alloy stock that runs through the rudder and is fitted with 'ribs', as in your photo...
The two side mouldings were removed to be repaired and the 'infill' that you see in your photo discarded...
Once the GRP side moulding were repaired one side was laid on a flat bench, the now clean stock was laid onto/into it and filled with an epoxy foam..
The other side was then laid on to it and the excess foam cleaned off..
Once cured the joint was glassed over to reinforce it, and once the rudder was cleaned up it was reinstalled...
The work was all done locally..
The only difference in your case, as I understand it, is that your Port side now 'sleeps with the fishes' so to speak, so a new one would require to be fabricated...
This was not at all expensive and entirely doable....