As an aortic valve patient myself, I think diet pills would be a very bad idea. Ask your cardiologist, but be prepared for the answer. I can't imagine that any cardiologist would be happy about an aortic stenosis patient taking diet pills. Losing weight if they were obese, yes. Taking speed, no.
Treatment of aortic stenosis depends on your symptoms.
No, do yourself a favor : change your diet and exercise.