You don’t say how old the lumber is or what area you are from. Wood identification is difficult enough when you can hold the item in your hand, weigh it, smell it, an judge the strength through handling, especially not knowing the history. Having said that, different woods were traditionally used in different areas of the world and country. Cypress was commonly used in the deep southern United States and resinous southern yellow pine (SYP) was used throughout much of the eastern United States. Based on that, my best guess would be that this is SYP. It looks a little like poplar in the photo but poplar would not hold up well to any exposure to the weather.
I have to wonder if this comes from a covered deck/porch because I can’t imagine a professional using tongue and groove material on an exposed deck since those features would tend to collect and hold water.
Unless you are doing historical restoration, the most practical solution would be to do as you say, just take pressure treated material and mill it as a replacement. Just keep in mind that pressure treated lumber tends to have a high moisture content and tends to shrink quite a bit. There are other solutions but they would probably end up being considerably more expensive.