Take the head off and remove the back plate. Spray the piston and sleeve with WD40, spray the bearings, con rod, and carb with WD40 as well. Let it sit overnight and she should free up. Do not turn the engine over at this point, as any stray rust and debris will surely scratch the piston and sleeve. Once free you will need to disassemble the engine to clean any left over gook and dirt that might be in there, I use denatured alchohol for this. Inspect the piston and sleeve for scratches, check bearings for any rust. Replace any parts that look worn or rusty. If castor oil was in the fuel you may have deposits that can look like rust, after soaking overnight in the alcohol these deposits will be gone, any stains or pits in the metal remaining would then be rust. Most of the time the engine isn't actually siezed, but rather the oil has dried up into a sticky gluey mess that makes the engine appear to be seized. I recently went through this with an engine that sat for 15 years with no after run oil in it.
The only parts I needed to replace were the crank bearings due to a small amount of rust there. Engine runs great. As a previous poster said, heat will help to free things up as well.  Hope this helps.