Welcome to the forum!
Change gearing is the best mod.
You can check out the Sprockets and Chain section for ideas.
Cheapest is going to a 12T front sprocket.
Then AIS removal and open up the airbox.
check out the mods here http://rickramsey.net/WR250Rmods.htm
Then FMF muffler and FMF Fuel controler.
On my 2012 I did the
Removed Flapper in the air box
FMF muffler and Fuel Controler
Race Tech Gold Valves Front and Rear