I love the way you add detail but preserve simplicity. You read a lot of personality into the generally stiff sprites. The originals succeed with the distinctive sprite silhouettes, a necessity with the old specs.
Your sprites seem overly rounded out -- to the point of being plush toys. Adding a few sharp corners would help greatly. It would give the shapes more contrast and add more visual interest. Maybe sharpen up some of the missiles, too.
Great work!