If that is what I think it is.. the rudder is a solid piece of plastic and the stock was chosen for the foil shape - but it wasn’t wide enough to fit in the chock so there are a couple of plastic cheeks plastic welded onto the main plastic rudder body.
If its what I think.. you possibly could just glue that cheek back in place, maybe plastic weld around the top corners? I’m not sure what type of glue however.. maybe 5200 might work or even JB weld - I’m guessing on the glue and its possible that no glue will work???
If you did repair, leave the bottom of the chock "open" like you should see on the other side so that it doesnt create a "line load" at the interface.
If you use the old one, drill some holes at the bottom of the rudder so that if it breaks at the top, you can flip it upside down and still get home.
If your old one is still useable and you return the new one, hopefully you don’t mind paying a fair re-stocking fee. We would like to see the rudder guys stay in business.