Welcome.
There is a bike salvage yard in south eastern MI. I can't remember the name or website right now, but that might be a good place to start.
The transmission is built into the engine cases on these bikes, so while replacing the engine you are also replacing the transmission. You do not need to get 2 separate parts.
There was not a '79 CB550. They were made until 1978, and in 1979 the CB650 came out to replace them.