The former 912E owner Chris Nicol on the board indicated the front calipers are 923 (912E specific) while the rears are common/interchangeable. A common conversion is to 911 fronts (rotors + calipers) to get around the limited parts availablity/$$$ of 912E specific parts.
Post a message on the
www.socal912.org board about muffler modication - apparently it is possible and looks very stock. I believe Bob did it recently.
Best wishes - Scott, 69 & 76 912/E