If you haven't already, you should read the thread Cat has a heart murmur
Only you can decided what to do. I presently have a 4 y.o. cat that has a grade 2 heart murmur. So far she doesn't show any sign of it, and runs and plays hard.
I did have a friend who had a kitten who had a grade 5 murmur, had him neutered, and her vet said he wouldn't last long, but he lived to be 15 yrs. old.
So who knows? Good luck