Little Echo had a similar heart murmur. We followed through with an echocardiogram (hence the name Echo) to find out if it was fixable, it was not. Echo passed away at 8 month's old almost a year ago. Echo would go back and forth to work with me (I work at a vet's) so that way if he needed treatment for something he would not be stressed by the car ride and being at the vet's. He had always had excercise intolerance (he would watch the other kittens play), but as he kept growing it got worse, even a scratching in the litterbox would wind him in the end. With Echo we never worried about the rules the other cats had to follow, no scratching the furniture, no people food (he loved Subway meat and Arby's roast beef (in small amounts of course)).
I would reccomend (if it is feasable for you) to follow through with an echocardiogram to find out how bad it is. We did do this with Echo under anesthesia (gas only) and neutered him at the same time. While the kitten is younger and smaller the better, because with his small size the heart doesn't have to work as hard, but as he grow there is more body to pump blood to. Keep us posted.