Cat Stolen still CO registered

Catalytic converter stolen from gen 1 Prius. They took everything except the muffler and I currently reside in a less than mod friendly state. The car is still registered in Colorado so it would be to there standards. My question is what do I need to get the cat replaced. Looked online to find cheap alternatives that I can have friends ship from Colorado but confused on if a single cat will replace the whole system. Trying to get his figured out!!!