You are in good hands with SCM.
There is no better way to put on some healthy muscle mass than by feeding your cat's raw.
Just a note on this link and raw feeding:
If you can't get thyroid glands, the add a tiny pinch of iodized salt for iodine and it is necessary to include some skin in the raw as cats do not get their Vit D from the sun, but from the skin of their prey
They mention heart being part of the "muscle meat group", but I feel it should be added regularly to your cat's diet as it contains a lot of taurine, an essential amino acid.
Cat maid to:
Puddles, female Himalayan (16 yrs)
Jasper, male Ragdoll ?? (approx 9 yrs)
Sweet Pea, female silver tabby DSH (approx 7 yrs)
Rose semi feral, a cpietra rescue, female tabby (approx 6 yrs)
Snowball RIP (1991-2005)
In a cat's eye, all things belong to cats.-English Proverb
“While we are free to choose our actions, we are not free to choose the consequences of our actions.” Stephen R. Covey