I have a SJ 26. The SJ 21 is a transom mounted rudder, the SJ 26 is not. I don't see where what you are suggesting will work. However, the SJ 24's rudder uses the same type of mount, you might try that.
Sorry it took so long to get back with you . Paper work was on the boat. My rudder is off the transom Loa specs is 25'11'"lwl 21'8" . My rudder broke because the previous owner made it a
swing , by the was its a 1977 . Any idea where I might find one or one that would work,