Further to MartinF's Coke idea:
For a while my Dad was restoring vintage tractors in NZ. His first, a Farmall F12, was seized solid after sitting for decades under a Macracapa tree (the vertical exhaust having filled the engine with water). a couple of 2 liter bottles of Coke down the barrel, wait a week, then tow it down the road dumping the clutch in top gear. Hey presto! it freed. Years later it still runs like a dream I don't think he has ever even lifted the head of it to check the condition.
Mind you it didn't have transfer ports, and it is a tractor..Err maybe just heat it and hit it as others have suggested.
Cheers and good luck