You will have better luck trying to turn the flywheel than tapping the pistons.  It’s near or at top dead center and rods have no leverage on the crankshaft.

Looks to me like someone did some soldering in those pistons.  I would reccomend dissassembling enough of the powerhead so you can access the other end of the pistons and look for rust in the cylinder wall from that side.  You don’t want to push a piston onto a rusty section of cylinder wall. You could also disconnect the connecting rods from the crankshaft and see which piston is seized, or if it’s a bearing that’s seazed instead.  After aplying more penetrating oil, you can also try to shock a seized piston free (via a suitable wood block) from the underside.  That way there shouldn’t be any chance of microscopically widening the piston and further wedging it is place as the geometry of the piston should make it stretch out if you tapped from the bottom.  Excessive hitting from the top may widen the piston even more into the cylinder wall.  Think like pushing down on a ball of cookie dough and flattening it.  Its width increases.  However, many engines have been unseized from the top side too.