I'm newbie to this post, so pardon my questions:
1. Like PeWe said: don't try to shift without also moving the countersprocket/shaft by hand (and-and-forth) between gears, or the gears end up dog-against-dog and won't let the shifter drum turn. This constant-mesh gearbox requires at least one shaft be moving (turning) during the shifts.
2. Start by getting all the way 'down' to 1st gear. Then search for Neutral, then work up and turn the countershaft during each shift attempt.
3. The 4-5 shift will be stiff: it is moving the gear with the widest grip on the mainshaft. If there is some rust on it from sitting or the like, it will be stiff, indeed! This can also happen if the engine has not been run in a long time, as the oil drained away from the mainshaft while sitting.
The K0-K3 gearbox has a narrow Neutral slot, too. They are also famous for bent "L" shift forks from incomplete clutch disengagement, primarily because they so willingly shifted 1-N-2 with barely a pull of the lever. Over time, this loosens the screw in the center of the shifter drum, then it will not shift into 2nd reliably. Then somone gets in there, discovers the drum's screw has backed out, and takes it off. The pins all fall out, and they have no idea how they were supposed to go in there: then the short one gets put into the wrong hole and one of the gears "diappears". I did this myself once to a K4 and it would not shift into 4th gear, going up. The short pin was in the wrong hole.
