i dont know if you tried the obvious, but once after changing to a aftermarket shift lever my bike wouldnt shift either so im thinking what the heck, then after i got off the bike and examined the problem a bit closer i saw the shift lever had been hitting the frame not letting it shift into higher gears, just a reajustment in the shift lever and things were ok, so check to if your shift lever may be bent, comming in to contact with something first, also if its a factory shift lever they are notorius for stress cracking right at the weld above the bolt, if it is loose on the shaft it will also mimic this problem, just some easy things to try first before you have to tear into the engine.