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.