The first step is to get the serial number, located on the top LEFT of the rear face of the steering clutch housing - the face where the drawbar or winch is attached. Or, on the right-hand (as you sit in the seat) rear of the engine. A small silver and black tag will have this. If gone, look closely on the steering clutch housing, it is often stamped near where the tag once was.
If this in fact a D9D, the serial number will start with 18A, 19A, or 34A.
Than, take this to your Caterpillar dealer and they can see what the availability for a new part, as used part, or another source. While you are there, ask about gaskets, studs, and nuts when you replace the manifold. It would also be a good idea to take a few pictures with your phone of the piece(s) you would like to replace.
I believe you will find these steps enjoyable.