It's been a long time since I've seen one of those, but it will be as Gw says and something in the shift mechanism rather than the clutch. First off I'd check there isn't some lever or cable that has to be used to put it into reverse.. That part might be playing up and stopping the shift lever from moving.. perhaps. If it's not a designed shift lock, then I'd try rocking it backwards and forwards while trying to get the shift lever to move. Sometimes we have to push the bike quite hard in one direction or the other to get the engine to turn and allow a shift to happen. Decpmpressing the engine will help, or, jack the back wheels up and turn them while trying to move the shift lever. If that doesn't work, describe how the shift lever behaves/feels.. is it solid and not move at all, does it move freely but do nothing, does it move the usual distance both ways, does it self center ?