Wow. I think that this a very nice drawing that you did and I can't think of a darn thing that would improve it.
I usually advise people (myself included) not to draw or paint someone with their mouth open and teeth showing. However, you did it and did it well...congratulations on a great drawing.
...Bummer that you didn't take the original photograph or you could soak up ALL the well-deserved glory for this portrait!