1976 Kawasaki KZ900 Wont Shift into gear!!!?

Alright, so as far as I was told by a friend, the bike will start fine and the clutch will engage and disengage...well..from what he explains it as, pulling on the clutch lever will give him some feedback making it feel as if there is still some friction there. However, when he goes to put the bike into 1st gear from neutral, it wont click into gear..it wont click into ANY GEAR for that matter. The shifter will move up and down enough to switch gears, but just wont click into place. Also what he has told me is that he was on it and out of nowhere, it just gave out on him. It didnt shut down or anything, it just didnt go anymore after he put the bike in neutral. it just stayed there permenantly. What I am guessing is that the Transmission may have gone bad already (I mean come on..its almost 35 years old and been ridden a lot) but I may be wrong..Anything I should lookout for? I just want some helpful pointers to try and fix the problem. Or just to try and troubleshoot this and see what is for sure that needs to be replaced, if possible. So please help me out! Any help is appreciated!!!