The damage is done, you're the new owner of the problem. I wouldn't expect help from anyone on this.
The cure for this is something you're not going to like. You need to pull the underbelly cover, replace all of the ducts, and I think you'll find the insulation is bad, too. Mr. rat didn't know he only was to stay in one spot and do his business in a certain location. If the rats decided to chew through a duct, they will have moved entirely through the under belly area, may have chewed through floors or cabinet walls, too.
Sorry to hear of your problems but it's doable to fix the damage.