wheel assembly for sliding screen doors

The 2 sliding screen doors are 18" x 76" each, and are adjacent to french doors. They have no brand or model info on them, and are at least 20 years old. One of the assemblies has failed -- the plastic pins holding the metal wheel have broken. These assemblies are internal to the doors. The screw protrudes on the interior of the frame, for purpose of tension adjustment. Only the metal wheel extends on the exterior of the frame, and fits into the ridge of the guide track on top and bottom.