Has the impeller been changed by any chance too ? Sounds like the gearbox has recently been removed
Sounds like it's not been put back together in the right place for the gears. Drop the gearbox and get it back into Neutral (can check the gears engage while your there) then refit making sure the top end is in Neutral too
Peter @ Boatsandoutboards4sale ~
askboatsandoutboards4sale@sky.com ~ 07930 421007