Any free flathead is a good one

Usually one of two causes of one stuck piston - either the engine was left sitting with the carb/air cleaner removed and moisture (in the form of rain or humidity) got in thru the intake manifold OR the block is cracked in that cylinder. When you get it out check carefully.

The suggestions you have so far should loosen it. In severe cases I've already used punches to bust up pistons to get them out in pieces. Last resort kinda thing.

Oil pump - they can stick pretty tight. Lots of penetrating oil from top and bottom and patience. I prefer PB Blaster. You can get to the top end by removing the cover plate on the back of the block where the drive gear meshes with the cam gear.

The tolerances around the shaft housing are pretty close and its fine sludge that got inbetween over the years that you're trying to get back out. One suggestion I read about was to turn the block upside down, hook a chain around the pump and lift the block off the ground about 1/4" and let the weight of the block dislodge it. The housing is cast so if you hammer on it you'll break it.