Buy a new primary cat?

So I just got my Genesis couple back from getting the engine replaced. Bought it with a blown engine so it’s my first time driving it back. It would tug and idle a bit and I got some few codes. Main one I’m trying to work on is P0420. Shop mentioned the car had no primary cat. I’d like some help here if I should buy a primary cat or if there’s another better route that will make the car sound nicer but also get rid of the P0420 code?