Motorcycle Repair: Km100 shifting problems, hook arm, double hook

Question
I have a 1978 Kawasaki KM100. my friend rode this bike for a while until the flywheel got loose then eventually broke the key. i replaced it and started it up but then noticed that the  bike diddnt shift at all. i took the cases apart and found nothing wrong with the gears or dogs. i was wondering if the case is warped or something... I have no idea... i will greatly appreciate it if you have any suggestions.
thank you,
Hiroki

Answer
Hiroki,

the way it works is when you push
your gear shift pedal it moves
a shifter arm shaped a bit like a little
double hook. This is usually under the clutch cover.
It has a return spring on it so it
can act like a rachet and keep centering itself.

These hooked ends catch on some
pins on the end of the shift drum.

This causes the drum to rotate and
it moves the shift forks in the
transmission.

The shift forks move the gears into the various
locations to create the  different gear speeds.

If it won't shift, it could be the shifter "hook" arm
or spring.
Shift drum problem.
It could be bent shifter forks or
broken or bad gear dogs.
It could also be the transmission assembled wrong.
If it is hard to shift it might
just be a faulty clutch.

Check here for some diagrams:
http://www.cmsnl.com/kawasaki-1978-km100-a3_model15290/partslist/