I just replaced the carpet in my 85. That particular piece has no visible edges, so you could get some carpet (original Sliverknit or some other lightweight carpet) and replace.
To remove the old piece, you would need to remove the floorboard around the pedals (one bolt), and the carpet-covered footwell blower covers (three screws). Remove the door-sill trim, then peel back the carpet section over the door sill enough to free up the piece in question.
You can use the old piece as a template for the new one. Glue it down with spray automotive carpet adhesive (I like Permatex). Re-glue the sill carpet, replace the other parts and you're done.
Here is a picture of the carpet piece in question in my car.
Here is mine when done. I added an R/RSR replica dead pedal to avoid the problem in the future. Works great.
--Dave