I was pretty lucky in that the carpet itself hadn't become moldy. It was the pad that made the entire house smell like a litter box. After we pulled the carpet back and dried out the pad, it was too late for the pad, and we had to replace just that section. If you caught it early enough, and it didn't start to mold in the sub-flooring you might be ok. Our subflooring was cement, so no problem. The carpet gets a lot of air flow through the weave, but the padding is what gets really nasty when it's wet. You can probably get away with just removing that large section of padding and put a new piece down. After the carpet is all dried out, just lay it back down and staple it back in place and you should be good. You might want to spray it well with Fabreze or do what I did and coat it with a box of baking soda and let it sit for a day, then vac it off. My carpet is white, and you'd never know that my tank leaked all over it.