Yardman self propelled

Hi all, I am trying to fix a Yardman ... model # 12AV829C755 ,21" mower for a neighbour, with the drive belt off, the transmission pulley spins freely, I'm pretty sure that this indicates a broken shaft,or key .... yes ?.I am reluctant to delve any further into this issue. Is the pulley keyed to a shaft or perhaps a set screw has come loose :)
Thanks.