Diet for an undernourished filly

Hello, I'm going to be picking up my Perch cross filly tomorrow and I'm so excited! She is underweight and in need of good groceries in order to continue to improve. The diet that the rescue has had her on is this:
Equine Senior, Calf manna, coccosoya and plenty of hay and good water. I'm wondering if anyone could give me some good constructive criticism or other ideas as to what might be good to add also or in place of these things? Thanks for your help!

How old is she?? I would start her on a ration balancer and rice bran removign the grains from her diet and adding fat