About the 912E
The exhaust system is one that is unique to the 912E. Thermal reactors (part #22 below) were required in California to meet the strict smog regulations there. Usually you will find that the thermal reactors have been removed and replaced with bypass tubes. Apparently the build-up of heat shortened engine life. You may have to replace them to pass a smog check, although some owners have reported passing it without them.
Stock mufflers are NLA, as are the heat exchangers, but NOS and used units pop-up now and again in online classifieds and auction sites. There was an attempt by a group of owners to have the Canadian company Stebro make a stainless steel reproduction, but most had major issues with the fitment. These units appear on eBay from time to time, but be warned, they may require some work before they will fit. They also do not have the port for the EGR filter.
Most owners buy a Bursch muffler, because nothing else is available. Although occasionally somebody finds OEM mufflers... but they are rare, and expensive. If you can find one...
You can buy headers from Performance Products for about $250 new or look around for some used ones. The stock muffler is no longer available but you can buy a Bursch setup for under $400. If your gaskets are still good after disassembly hold on to themthey are NLA also. Copper exhaust gaskets are available at the Type IV Store.