If the engine is seized by rust in the pistons you can pull the plugs and pour in some WD-40 or any other good rust buster. I use a brand called Sea-Foam but it is not available everywhere yet.

By the way, it is not the piston that will be rusted to the block but the piston rings. This is a small point, but the reality of this is that you might have to keep pouring the WD-40 in for a few days. You can also use automatic transmission fluid to bust rust if you have an old bottle of that laying around.

So how can you tell if it is rusted or busted? If it will just not move at all it is likely rust. If it will move a little and then hits a hard stop it is a mechanical problem.

You might also pull the pan and look for a broken rod bearing cap. Usually the bolts holding the cap break and allow the cap to move and when this takes place there is a HARD slam inside the block. FYI: If you find this to be the case then the crank is likely bent so if anyone plans on reusing it they should have it checked first by a shop that can do this.