Tony,
I would have the head off first to see how bad the corrosion is, if not too bad then diesel is good as it has a searching action and is a lubricant. I have had gudgeon pins siezed / rusted, which does not help either. With the head off and a lump of hard wood, you can thump the pistons alternatively to rock it back and forth slightly to break any rust bond on the rings. It may be too badly siexed, like an Austin K2 engine I rebuilt a while back, now that one, I had to take the crank out and really force the pistons down, that one was so bad it definitely needed reboring.
Coke contains a mild dose of phosphoric acid, we used to have large tanks of this acid to clean engine components on the overhaul line, it eats any thing like alloy though. Thinking of it, Coke comes in alloy cans so the acid strength must be quite a bit mild :-D