I might race XC on my 300ex october 1st.By Then I will have a supertrapp IDS2 exhaust and a K&N filter.I already have the 20/11/9 6 ply razr's on ITP .190 wheel.I had them on my 250r but since i will only be racing mx with it decided to keep them on the ex.What other mods do I need????The XC is a missouri harescramble in festus missouri which where I live so I figure why not.

almost full race 250r
moded 87 LT500R
97 300ex
90 blaster
96 warrior

 

I wouldn't do anything else as far as the motor is concerned, but I would look into some shocks.
The better it handles, the faster you can go.

 

weisco, and porting (valve job), all helps torque (low end-midrange)