Acre Garden, I think Winston was asking about the potential for water getting into the engine as a diagnostic thing, not to judge your storage methods. If motor has probably gotten into the engine, the procedure to fix it will go differently than if the clutch disk is frozen and the tractor is in gear. (Just in case, make sure you're in neutral.)
If the exhaust gets water into it, sometimes it drips down into the cylinders and sits. Check the oil before you do much else. If it is up above normal, there is definitely water in the lower end. Over time (and it doesn't take that long) the rings and cylinder walls rust together, and stick. Pulling the injectors will give you a direct shot into the cylinders. Spraying a penetrating oil in there can sometimes loosen up the rust, and let the engine move.
The injectors have two bolts on a retaining plate that fasten them down,
like 17 in this picture , though this is off the next smaller tractor than yours.
Sometimes the injectors are impossible to remove without a puller. Don't try to use vise-grips to mangle it out. You can rent the pullers from Hoye; other places locally may have them too. You may have to pull the cylinder head, which isn't that hard, to clean out the rust. Before trying that, if the injectors won't come out, pull off the exhaust manifold. You can probably thread the straw of your oil spray up the exhaust port.
With the valve cover off, press down on the exhaust valve stems one at a time and blast away. Let it settle in, repeat. Then give the oil time to work. It may give you enough to break the engine free.
Good luck. These are excellent little machines, and relatively easy to work on. You'll be able to get through it.