As other have said, they may grow out of it and even if they don't it might not be a big problem. When we adopted our two rescues we were told that one had a heart murmur (think it is the lowest grade) but that it wouldn't affect his day to day life. A vet might have to be a bit more careful with him under anaesthetic but given he had already been neutered it clearly wasn't that much of an issue as he had obviously managed to cope with that op. They were 18 months at that time and we've now had them over a year and I have yet to see it hold him back at all, he is as hyper as any other cat when he wants to be.

Agree with other posts. Just to add that once the anaemia resolves ( and it will because this is a regenerative anaemia ) the heart murmur should also cease.