I think a little more history is in order here, like what happened since the last time it ran? Has it been sitting for three years outside with the carbs off, or is this something new since it was ridden last weekend? Were there any weird noises or indication something was wrong last time it was running?
It's highly unlikely the trans or final drive are locked up, but possible. Also, the G/S having nikasil cylinders isn't prone to the rings rusting to the bore. Perhaps the brake shoes have bonded to the drum? Sometimes one of the U-Joint bolts will work loose and jam the driveshaft.
So what happens when the motor is cranked with clutch in? I don't believe the kickstart will turn over the motor with the clutch in, but if the kicker works, that would tell you the trans is free. You can also pull the rear wheel through a rotation with the trans in neutral, and would tell you if the trans and FD are ok.
Wanted: Dead, smashed, crashed or trashed gauges
BMW GAUGE REPAIRS - TACH*SPEEDO*CLOCK*VOLT METER *PODs & LIGHT BOARD*