Putting together my new (30 yr old) sunfish. The rudder pin was corroded in place, but some WD40 has freed everything up. Part of the pin is bent, but that's not my problem.
How does the rudder pin not fall out? Is it welded to the spring? Is there a cir-clip / snap ring / lock washer in the groove? I have a pin and a spring, and the pin has a small groove about 1/3 of the way down that appears to be for a snap ring or other device to hold the spring.
I take it you are refering to the C clip retaining ring. Here's a breakdown on the rudder parts http://www.layline.com/llf/Boats/sunfish/blades.asp
If you're buying locally, measure the diameter of the pin, not the groove to get the size required and remember steel rusts and you should get a stainless C clip.
Check out Sailing Texas instructions for replacing the C-clip (http://www.sailingtexas.com/chowtoreplacepintle.html). Get a couple of stainless steel clips, I believe they come (from Vanguard) in 5 paks, but you need to check.