Hi Jens,
Welcome to the forum.
While there are no proper drawings with measurements of the standard SeaHawk rudder on the web site, you should be able to scale up the photograph you see at:
https://www.seahawk17.org.uk/boat-description-keel.php#rudderHowever, depending where you sail, you may think that some of the modifications tried by other owners offer a better alternative. See the page at:
https://www.seahawk17.org.uk/owners-custom-sailing-rudder.phpIt might also be worth looking at this forum post which informed some of that page:
https://forum.seahawk17.org.uk/Rudder-Modifications-td1903655.html