KX 250 1999 Crankcase question?

Have you tried slipping the shift lever on and try shifting through all the gears and neutral? Sometimes you have to move the main drive spline shaft with your hand to get the lever to shift into the next gear. In neutral you should be able to easily spin the main shaft by hand. I would start with that. If you can't shift or move the main shaft by hand, you better re split the case.