Carefully inspect the rear chassis legs all the way up to where the trailing arms attach to the chassis before making a decision. Use a pointy thing as part of your decision making.
I repaired the same area on my son's Ninety a couple of years ago maybe.
It now needs a new chassis, although I am still welding to give him some time to save up.
You are very lucky if that's all it needs.
A light tapping stick helps, when the 'ring' changes you know there's a hole/thin bit somewhere near ....