How to reduce the fat amount on a thin body type?

I'm 50 kg and 163 cm tall. I think I have the ideal weight compare to my height and age. I seldom eat unhealthy/fat food, I use almost no sugar and salt. I don't have a regular exercise plan but I do pilates and strength exercise when I have spare time. Even though I have muscles on my upper body, I also have a lot of fat, especially on the stomach part. I don't want to loose weight but I want to have a normal fat amount on my body. Is doing cardio exercise the only way or is there anything else I can do?