IMO, the best route to take would be to replace it since the hit from the rock may have also damaged the "honeycomb" elements inside it, plus re-welding may not be a easy task. RockAuto.ca should have a replacement cat that fits your budget.
It also looks like your missing the orginal subaru spring/bolt combo where the mid pipe and cat assembly bolt together, that is there to lessen exhaust vibration and stress. RockAuto should also have those plus the gaskets.
Yes you could use the stock cat back. I would just replace the rear cat, if your current cat assembly doesn't have a flange between the two cats then you just get a flange welded in and bolt on the new rear cat assembly as it will have flanges on both ends. The problem with driving the car around like this is that the exhaust gases won't be all vented out the rear so there might be some risk if gases entering into the car.