Have you heard about Co-Q10? It is one of the top nutrients for the heart. If the heart has the nutrients it needs, it can repair itself. I'm assuming that myocardial myopathy is the same as cardiomyopathy. In the early 80's, one of my cats was diagnosed with cardiomyopathy when he was about 7. I found out about Co-Q10, quite serendipitously through a health newsletter that my mom received. I started him on 30mg daily and by the 7th month, his heart had returned to normal size. The only remaining symptom was a murmur from a floppy valve. He lived to be 18. Back then CoQ10 was quite expensive and didn't come in large doses. Now you can get 100mg quite affordably. It now comes in the form of Ubiquinol which is more bio-available.
You should probably take at minimum 300mg Ubiquinol daily. If you can manage 300mg 2-3 times daily, that would be really good. Additionally, vitamin E, some magnesium would be good for the heart as well as hawthorne and cayenne. I'm sure when Dr. Mom answers, she'll have more advice regarding helpful herbs.