Hi Steve, some info about the machine, ie model: Series 1,2,3 etc. Engine: BJ, JAP etc would help to diagnose the problem. It could as you say be the main output shaft, this would mean the gearbox needed stripping and overhauling. It could equally be the clutch plate, as the plate has holes, into which locating dowls on the flywheel fit to take the drive. If that has broken up, then the engine would need to be removed. You could check the fault by removing spark plug if petrol, putting it into gear and turning the start handle. If the machine moves, I would say its the clutch plate. If no movement, I would say the main output shaft. If neither proves conclusive it could just be the starter dog failed. Hope this helps