It's probably rusted through so you'll need a new one.

It's a bit tricky to get over the exhaust and you may need to bend it a bit and/or drop the exhaust off the hangers to get it on but they're fairly cheap. You'll also need new hex nuts as they're not really reusable.

I wouldn't leave it off personally, you'll probably damage the underseal and transfer heat into the cabin when it gets really hot...not sure if it could start a fire but it may be a possibility.