Man, i've unstuck a bunch of motors in the last 2 years..every one of 'em with WD40.
Buy the stuff by the gallon, not the spray can.. take out the plugs, and fill the cylinders full with WD40.. then take off the valve cover(s) and really coat the heck out of everything.
Put the plugs back in, set the valve covers back in place, and let it sit and soak for about 6 hours.
Come back after six hours and check to see that all the cylinders are full of WD40.. if not, fill 'em back up, and put the plugs back in.
The idea here is to let the WD40 do it's work.. breaking up all that rust.. then after a few days of soaking, get under the vehicle with a breaker bar and try to rock the engine back and forth by putting your breaker bar on one of the flywheel bolts.. make sure the trucks wheels are blocked, the sparkplugs are out, and that the tranny is in neutral!
If she still won't budge, fill the cylinders with WD40 again, put the plugs back in, and let it sit another 24 hours and try again.
Some dudes reccomend PBBlaster, other folks have said Kerosene is the stuff to use.. regardless of your fluid of choice, the general idea is to break up any rust that is preventing the pistons from sliding up and down in the cylinders, as well as any rust that may be keeping valves from opening and closing, or inhibiting the free travel of the push rods, etc.
I don't think i'd use Coke.. just because whatever rust it eats will be replaced with gooey sugary muck.
I'm sure brake fluid would do the trick, too.. but brake fluid is pretty nasty stuff.. it might eat your piston rings right along with the rust.
