That looks very similar to the H23 rudder assembly. Can you take a pic of the rudder itself, that shows more of it's outline?
Assuming it is the same basic design, keep in mind the rudder, that obviously swings toward the rear, will want to float so it is horizontal with the bottom tip toward the rear. The line is used to hold it in the deployed (down) position. The rudder obviously is bolted inside the aluminum head using the bolt through the hole in the head and the rudder, so the bolt is sort of an axle letting the rudder swing up and down. The H23 rudder has a U shaped fitting attached to I think the rear upper edge (that is why a pic of the whole rudder would help). A line is tied to that, goes down inside the head, out through the sheave in the back side of the head, up and through the cam cleat above the sheave. You pull up on that line, through the jaws of the cleat, hauling the rear part of the floating rudder back and downwards so it swings into the deployed position, then lock it using the cam cleat. Maybe around a 3/8 line, I can't recall but look at the width of the sheave and cam to get an idea of what diameter. I used a line with a braided cover, not a 3 strand. If you hit an object the line gets shredded as it is pulled backward through the cam jaws as the rudder kicks up.
You may want to fix up the hole for the bolt through the rudder with some fiberglass. I found an aluminum tube with inside diameter just right for the bolt and glassed it into the hole, careful to keep it at right angles through the rudder, to reduce gouging of the glass. You may also want to have something welded to the head, or maybe enlarge the holes through the aluminum to fit some sort of insert, if the holes in it are reamed out.