I race enduros on my 04 KX 250 which I bought as a leftover at the end of last year. I have done the following to it, these mods will apply to pretty much any bike.
Big tank, Clarke or IMS
V-Force
9 ounce Steahly fly wheel weight
Skid plate (when I go to the rock runs)
Hand guards
E Line pipe guard
WER steering dampner
WER suspension revalve
Lights
ICO Checkmate computer
Ive used both the FMF Q and the Pro 296 silencers, both are quiet but I had durability issues with several Qs. In my opinion, the PC 296 is the way to go.
FMF Gnarly pipe, I do have a discontinued Enduro Engineering low end pipe that I havent tried yet
Of course some stuff in my list are enduro necessities and not needed for woods riding or hare scramble competition.