40 years ago I worked at a foreign car shop, and there was an XK-140 that looked as though it had sat in the woods for more many years (that’s because it had). The owner had to cut down trees to get it out!
Needless to say, the motor was frozen. The boss told me to pull the plugs and shoot some penetrating oil into the cylinders several times a week. After a few weeks I put it into 4th, sat on the bumper with my feet on the wall, and rocked it.
The rocking went on for weeks until it suddenly broke free. I changed the oil, flushed the tank, replaced the rubber fuel lines, did a quicky gasket-change rebuild of the SUs, flushed the cooling system and put in new antifreeze (surprisingly the cooling system was still intact and the coolant looked good). Before replacing the plugs I cranked the engine till it registered oil pressure. It started right up—and smoked enough to kill ever mosquito for miles. But as Rob points out, after a few more startups, the smoking settled down to normal vintage British levels.