For just the Lexx you probably could get away with the stock jetting, but I agree with the others; the KLX kit is the way to go. Go ahead and drill the airbox lid or do the L-mod (it does help flow with no real downside) but at the very least pull that snorkle!
Follow the instructions from EM on the jetting. My bikes both have the 4 - 1" holes, snorkle-ectomy, Uni/TwinAir, FMFQ4/LeoVince X3 and both run a 142 main, second position , no washer under and 2 turns on the fuel screw. Jetting is spot on at those settings; I live at 1200 ft above sea level and regularly ride to 5000 ft.