Chances are, all of your pistons have corroded on top,
and what you're seeing is that #2 piston happens to be close to top dead center and you're just looking at the top of it because it's at the top of its bore, while the other pistons are down somewhere else in their bores.
But by all means, get some oil in there and let it soak.
Something else I'd try would be to throw a little thermodynamics into it (after it soaks a few days), by taking a propane torch and heating up one side of each bore. Heat has a way of expanding and moving things around, and it might move just enough to cause a crack in the corrosion, allowing the oil to seep in. And it wouldn't hurt to tap on the bores with a piece of wood or plastic mallet. Sometimes shock waves help.
I once had an old 18hp Evinrude motor that sat upside down in the Mississippi river during the entire '93 flood. It was locked up too, but after a few days of oiling, heating, banging and tinkering, it freed up and I was able to get it running. Smokes a lot, but then, it always did. It's a 2 stroke. My uncle has it now, and still uses it.
