Dave_panda, correct me if i am wrong but i think FMF dose not make a GNARLY for the KX85.
83MX80, i have a 1979 IT175 wich is very similar to your bike. So, i will compare both those bike to your current one power wise and handling.
KX125, light, less bottom end stock, better suspesion, stalls more at low speeds, jumps fantastic, cuts the corrners very sharp, has lots of mid range and top end hit and power, your bike will out run it in its 6th gear pinned but not by much, the 125 will get there a whole lot faster, and you cant be lazy so you have to stay on the pipe.
KX250, still light, about 1 third more bottom end compared to your bike, this bike will eat yours in any gear, it has enough power to wheelie in 3rd gear with out any effort, carvs the corners, jumps better, suspesion will noy bottom near as much, all in all better in every way except price.