If the piston is seized, it's likely a ring, jammed/stuck/broke. Hopefully the jug is OK, but not likely- be prepared to replace the top end. I'd recomend opening the clutch cover, just to eliminate the slim chance that it's something potentially easy to fix....
I hope the best for your repair- keep us posted!
Could try a firm grip on the jug (or get a helper to get a hold of it), and try the wooden end of a mallet to rap/push (bang that sucker out!) against the piston-
-As an after thought, if you have the head off, and pull the clutch in.... does that give you any play to lift the jug and the piston stuck inside.... with rotation of the crankshaft there should be some movement?
If nothing I've said helps- do not be surprised.....=)
-Richard