Try pulling both steering clutch levers back at the same time while driving ,the tractor should stop as when they are both back they should disconnect the drive . If it stops both clutches are ok ,may be it has leaked oil into this area ,drop the drain plugs under both clutches and see how much oil comes out they should be dry, a lot of owners leave them out , if a litre of oil comes out ,flush with kero with plugs in drive for half a hour and drain ,test drive once kero has drained away.
Best of luck
Richard j