I race motocross my self and my first mods would be this.
1. Better front shocks, and possibly revalve rear one.
2. Pipes (need em to wake the motor up) I'd get the PTR mids or Dynoport 2:1 or FMF fatties.
3. Some better handle bars and better clutch lever. (MSR levers pull very easily and helps a lot to have an easy clutch pull.
4. Get it so your throttle pulls easier, (2into1 carbs are pricey but worth it.)
5. Better tires.
6. Wider a-arms and axle.
and of course the necessities such as nerfs, tether switch, and safety gear.
I would say get wider a-arms and axle first but if you have a budget problem then thats a bad route.
The best way to figure out what you need is to start racing, and change shit that you would like to improve on it.