Remove the starter and check if the gear can be moved. On my 521 the gear starts from an extended position and is pulled in by the motor to mesh with the flywheel teeth. The gears on those commonly rust or freeze so they will not pull in. Spray it with some solvent like WD-40 or PB-Blaster and let it set for a few hours. Then work a screwdrive between the top of the gear and the housing so see if you can get it loose.
