Change the oil and spark plug, New Battery, Adjust the points.
I don't know about the plug and points, but I know the oil is fresh and the battery is good(starts all his other riders no problem) Should be turning over easy seeing how most of the older kohlers had a ACRChange the oil and spark plug, New Battery, Adjust the points.
Nothing is hooked to the pto so I don't think its dragging, but it is hydro so maybe thats causing the problem.Is there PTO dragging at all? If its a Hydro..cold hydro fluid is thick and if the pump is not perfectly in neutral it can drag the starter down.