This may not help, but there is a kit of plastic sailplane control parts sold by Hobby-Lobby. Most of the parts are things like belcranks for full-flying stabs, but mayhap there is something in there that can be adapted. Since they import graupner models, you might also want to call them and ask if they can special order a couple of these specific parts for you.
Failing that, You could probably make a working replacement from glass cloth and carbon fiber and some epoxy. Another way to go is glue the snapped part back together with CA and then make a mold of it using one of those resin casting kits in the craft store. Then you could pull extras and maybe make back the cost of the molding kit replacing a few for friends that also broke this part:-)
