I think a 125cc 2stroke will be the best for you. If you can ride it well, be sure that you can beat 4 strokes in MX track. And smokers teach better brake-clutch control because they only give power at high rpm's, so you have to keep the engine at high rpm's with better control. And 2 strokes are extremely light compared to the others, especially the lightest is Kawasaki KX125 - only 87kgs.
I will get a 94' KX125 tomorrow, I'm 140lbs and age is 17. 125cc 2 strokes are the best bikes for us :nod: