If the cork seal has square corners, perhaps try to see if you can get a new modern material one the same shape made. For the A90 Atlantic hood and window piston seals, I have had these made at an Hydraulic Seals manufacturer. Although piston rod seals don’t rotate, and the steering peg does, it is not a fast rotating seal, and being slow turning, may well be able to be used for the application.
Some time ago, I did a repair of an Atlantic box, which is the same as an A70 casting, and it had part of the seal edge broken out. Being sufficient gap between the pitman arm and the bottom of the alloy box casting, I machined a small extension cap to take a modern seal, with the top section extending up the bottom of the casting slightly. In this extension, I used three pointed grub screws at 120 deg centers to secure it to the box. So with a new seal in the bottom of the extension, and some silicone gasket seal on the mating surface of the extension to the box, it was secured in position. Although nothing would be as good as machining a proper seal seat, even one of the purpose made seals might be sufficient to do the job. What is the actual size of the cork seal, and how is it held in position.