Rock the flywheel and see if it frees. If not, remove connecting rod at the bellcrank and then get a chain around the plunger head... tie it to something solid... and drive away.
I would suspect that what has happened... there is a big rust boil under the lower right guiderail in the bale chamber nearest the knife. This pushes the rail upwards and pinches the plunger in place. Force may be the only way it's going to move.