I don't know if this will work but might be worth a try.
Soak the piston with a 50/50 mix of acetone and ATF for several days.
Fab a cross brace of stout steel that will go across the bore between the two studs that are part of the cylinder.
Drill and tap a center hole for a large fine pitch threaded bolt in the center of the brace.
Bolt the cross brace to the cylinder at the studs.
Place a piece of stout aluminum between the center bolt and the piston top. Tighten the center bolt to drive the piston out of the bore.
Let us know if that works