ive built a race z/kfx400, get real race shocks all the way around any of the stockers out there are garbage, a-arms, get rid of the crap stock swinger, axle, quality 440 kit (high comp), have the head completely built to handle cams (stock valve springs and stems love breaking with any race cam) get an FCR carb (yfz fuel systems run cheap) dump the stock intake and build one with a pod system, steering stem, rear shock linkage, gusset the cap out of the frame, nerfs, full exhaust system, beadlock rims, steering stabiliser. the rest is all personal preference ... unfortunetly the z400 is probably one of the most expensive quads to prepare for mx racing