Sad and stranded sailor

This is my 3rd summer sailing my old, second-hand Sunfish, and I was just beginning really to get the hang of it.. and I love it! But my rudder cracked. So I found another old rudder, but it doesn't fit precisely. How do I know exactly what to get so that it works? And do I need a new one? Would a new one even fit?

Or...is there some miraculous substance you can use to fill the crack in the wood?

Please advise: I'm dying here on dry land!