Try not to worry I know it's easier said than done.
My Jersey(RB) had a heart murmur. I had to give him half a St Joseph asprin every day. He died at 12 but I think it was due to his overall not so good health (bronchitis, asthma, allergies). They did a catscan (Ironic I know!) and $900 later they told me it was a heart murmur.
I believe someone else here adopted a kitty with a heart murmur as well. Let me see if I can find the link.
It depends on how sever it is but I think that most of the time kitties do just fine with murmurs. I know sometimes they grow out of them. How old is Marius?
Extra skritches for him tonight...he's had a hard day!
Claudia