The centre on the piper is essentially a sleeve fitment, but it comes with an adapter to fit an olive joint set up too. So really any decat will fit. But you'll be better off getting a sleeve type decat.
Most sleeved ones will fit, ktec's one, milltek, and piper do have their own for the system.
When I was looking the milltek one was the cheapest option for it. I did find it droned a little a certain rpms low down and when applying some gas higher up around 50-60mph it got quite loud, but when cruising at 70 it was silent, 80mph was very drone and 90mph was silent lol. Although it was only 96.5db when I had mined measured at mallory so it wasn't overly loud just that droning that irritated me. I ended up with a silenced decat though due to mallorys tighter noise limits on track and it sounds great, bit louder than with a cat and no drone at all.