G'day wce,
What I'd do here, is to make up a brew of 50/50 by volume Acetone and Automatic Transmission Fluid [any type of ATF], and tip some down the spark plug hole - as much as will go in. Then prop the engine up, to keep the cylinder vertical, and leave it for a week or so.
This is a much favoured technique in the Vintage Engine Restoration community, for freeing up piston/bore seizures. And some of those engines come up OK, after decades of being seized...
