I finally received a reply from Amel:
Dear Mr. Grinius,
As for your bowthruster we cannot build a new lower unit as the parts do not exist anymore.
It is very difficult to take the bowthruster apart to get access to the gears located inside the unit.
The only way to get access to them is to slightly sand around to remove the GPR. Then you will find 2 flanges bolted together (white). Inside you will see the gearset (we have sent you the drawing in a previous email).
To fix the crack, you can sand all around the lower unit (few mm thick) and then redo the laminated strips.
With our kindest regards,
Maxime PELLERIN/Chantiers AMEL Customer service
It looks like the white plastic (?) ring is a split ring to house the seal. While I finally got the the seal to hold oil with no leaks, this component (bow thruster) will careful attention to keep it running. I also Amel if a Maramu's bow thruster would fit in the compartment.