I think your work is pretty good. You might work on getting rid of the lines in your picture.. that isn't in reality. The shading could also be a bit more worked. The difference between lights and darks is what will make your picture pop.
Not being able to see the original.. I don't know how the features are.. but it appears that the eyes are a bit far apart.. You might try to either put the picture over the original to check proportions, or lay them out digitally, or triangulate. These methods will all help keep the proportions correct. You might also try gridding if you find the features off a bit.