I can't recommend shifting it into gear without the clutch disengaged. That is a good way to damage the transmission. I would warm it up for about 10 minutes in neutral, shut the engine off, shift it into gear, pull I the clutch, and try rocking it back and forth really hard. You can also start it up in gear with the rear wheel off the ground, pull in the clutch, and apply the rear brake while trying to keep the engine running. Try putting some Seafoam in the oil, let the engine run a couple of minutes with the rear wheel off the ground, then let it soak for a while. You could put the front wheel against something solid, and try to start it, but you risk damaging the starter and starter drive that way. If all that fails, go ahead and pull the clutch cover. It's not a big job. The friction plates probably need replacing anyway. There could also be something wrong with the internal linkage.