Well, its looking pretty good to be honest. But one thing I need to ask, is it male or female? as the original concept is neither here nor there, but this has a female quality to it, which is never a bad thing. Girls are more intersting to have astride a bike.
Anyway blabbing aside, if its a female, then I'd narrow those hips down a bit more as the concept has very thin hips. Also take the hairline back a bit as the forehead is a little square. If you fell you wanna send me it to have a quick tinker please do, but send it as an .ma not .mb file or send an obj