Here was my old KDX250:
Ton of mods. But had the KX gaskets, wiseco piston, ported cylinder, Vforce reeds, KX forks, Ohlin's shock, 19" rear wheel
This is the jetting that made the bike RIP at sealevel and on amsoil 50:1 mix:
Jetting: 38/158 , R1367H needle 3rd clip from top, #6 slide
Basically you need the KX head gasket, good exhaust, remove the airbox lid and the needles and slide. Then if you are not running 50:1, probably go up on the main and pilot a size or 2. But that jetting made the bike scream!
Sold it when I went fulltime MX. Good times though!