If it was running when parked, it likely is not stuck very hard. I agree with putting some oil or Marvel Mystery oil in the cylinders and letting it soak a few days. You could also simply add some diesel fuel to each of the cylinders as it is thinner.

I would use hydraulic pressure as a last resort as that can do major damage, and it should never be used on a piston that is a top dead center or at the bottom of the stroke (no leverage on the crankshaft and can bend the rod). Instead, remove the starter and use a pry bar on the the starter teeth. You can get a massive amout of leverage there. Don't worry about breaking a tooth, as they are a lot stronger than you would think. Just don't pry on only a corner, but get as much of the bar on the tooth as you can.

I would definitely try to get the unstuck and engine running before considering disassembly. Any damage will already be done once it is unstuck, then you can see if there really is any damage.