Yes you will want to open things up some and install a two to the three or whatever is there with a combination fitting or a Y and an 1/8 bend. You will need a vent for which I would also use a combination fitting. It appears that you are starting to get some dry rot in your sole plates which appear to be untreated fir. You will want pressure treated lumber there if you can do it, although it would be difficult and labor intensive at this point. Maybe theres a vapor barrier under there. The good news is that your floor is not too thick, and should be easy to break out. (If you could call that good news) I dont see any clean stone though, which is kind of standard under a slab. Im sure you know this, but shoveling the dirt out and undermining the slab will make removal with a sledge hammer much easier.
Good Luck, John Henry.