Chewing can be due to various reasons

Lack of food and water, gap between the tubs and frame etc.

You need to make sure that the part where the hole is in the pan is not accessive (it needs to be pushed back under the frame as much as possible)

You also need to provide your rats something to chew on (a piece of 2x4 will do)

You need to get rid of all the rats that were in that tub, rats that know how to chew will try again and even worse they will teach others therefore they need to be fed of.

Now as far as the damage I would simply replace the tub.