So have you bought this kx100 or not as in your other question you state that you are looking for a pipe for the kx100 you just bought. you have not stated how old you are so that makes it a bit harder to advise you of what bike but for your size and height you really don't have too many options. Either a 2 stroke 125cc or a 4 stroke 250f would suit your height. You could ride the crf150r but are about as tall you anyone who manages to fit on one and may find it a bit cramped. Most serious racers these days opt for the 4 stroke 250f's as they are the more competitive option. If you are as good as you say I am surprised you don't know what the next step would be.