Heat it up real good - this should loosen it up well. Soak in some acetone to loosen up the bearings. Rinse thoroughly, dry well, and oil it. Being a ringed engine, the ring is seated to the liner. Don't needlessly disassemble it if the bearings are smooth after cleaning and oiling. Removing the liner will upset the ring seating and require some careful running to re-seat it. It's usually best to free everything up and run the engine to determine if anything needs to be done.

And lastly - DONT use vice grips or other malicious tools to turn the engine over! This includes putting the engine in a vice as well! Tool marks will turn off a lot of potential buyers (should you decide to sell it) as this will show inexperienced hands have had hands on it and may cause other damage to the engine. If you're not sure what you're doing, ask questions first.