Oh, that is so sad and as others have said, don't give up hope. Last month my 4 month old Cinder was diagnosed with a heart murmur too - the vet didn't give it a stage number but said it was not very pronounced and that it's common in kittens and she will likely grow out of it. My husband even had one as a kid and grew out of it completely. Your kitty is in our thoughts and good intentions are being sent out as I write this and read this thread.
I wish you and yours the very best!