Worst comes to worst, if you cant get them out, just take a grinder or small cut off wheel to the heads of the bolts and the valve cover should come off. One time I snapped one of my valve cover bolts in half, leaving the threaded end stuck in the head. I didnt want to hassle with drilling and retapping, so I just took the head to a machine shop and they removed the broken end and put in a helicoil insert. It only cost me 20 or 25 dollars I believe. My suggestion is to do that if you want a quick, relatively cheap fix.
00' 400EX - GT Thunder 450 stroker, custom JE 12.5:1 piston,
+7mm stroked crank, HD stroker rod, HRC cam, GTT full race porting, +1mm intake valves, FCR 39 carb, CRF timing chain, Denso IXG27 iridium plug, full Hinson 8 plate clutch, Desert Toyz Supercooler w/ SPAL fan, UM airbox w/ UNI, CT Hi-Output exhaust, Roll LOBO II F/R, LT PEP 8 click ZPS piggyback fronts, PEP ZPS rear, YFZ brakes/450R master, CCP stabilizer, Hiper Tech beadlocks, Durablue X-33, TAG, Quadtech, ASV, AC, IMS, RAD, Works Connection, Galfer..
05' Kawasaki KLX 110 - Fast 50's springs F/R, HRC cradle, Rox risers, FLY fatbars, NGK CR6HIX iridium plug, Twin Air filter