How to free up stuck piston on toro 21" front wheel drive mower

My brother passed and I inherited his collection of lawn mowers that he used for his 1-man show business. The mower in question was loosely covered with a plastic bag and left outside for 2 years in NW Pa. I am in the process of evaluating if I can get it running again and discovered that the piston seems to be stuck in the cylinder. I tried to move it by rotating the blade but it didn't move and I didn't want to force it.

How best to free the piston?

I thought about removing the spark plug and spraying some 10w30 oil in the cylinder or using some PBlaster but not sure if that will cause any damage. Thought I'd ask the pro's who probably have run into this situation.
He also has a similar mower with a B&S engine in the same condition. Both mowers have the correct amount of oil in them.
BTW, AFAIK, the mowers ran before he 'shelved' it.
Any guidance appreciated. Thanks

If it came to me that way I would spray a lot of PBlaster into the cylinder, stand it on end with the spark plug up, making sure liquid gets to all sides of the piston. When I’m sure set it down and wait for 2 days for the PBplaster to do its job. Now I would put a 5/8” six point socket on my 1/2” breaker bar a slowly try to tighten the blade bolt. You must do this carefully, as you don’t want to break the bolt or shear the threads. Patience is your friend and sometimes a big hammer helps if you know how to use it to help break it free.

You would think if he bagged it up it was a runner at one time. Bag held moisture and rusted the rings up is my guess. ????

Citric acid is a Mercedes Benz approved trick on radiators. It is very gentle. Rust converters, naval jelly, acetone and ATF or a heat gun. Several ways to do this. I would leave hammers as a last resort if it was mine. Air hammer striking a wooden block in the bore. Combo of all these.......

pull the spark plug and make sure that the issue is not fluid in the cylinder. If it still does not move, use some wd-40 and PB Blaster in it, enough to soak around the edge of the piston, and let it sit a couple days. Once it does turn, you'll need to get a bore camera in thru the spark plug hole to make sure that the cylinder walls are still in good shape. Rings and carbon can get stuck just hanging around in moist air. Valves can stick too - meaning a piston can be stopped by a stuck open valve. Also the flywheel can rust a little and the coil stick against the flywheel. Anything is possible.

If it is suspected carbon problem why no use a de-carbonizing fluid.

But as said Post #4 anything is possible. Even things that some say is impossible.

It's odd an aluminum piston would seize in an aluminum cylinder with oil in the engine. Usually i see engines stored outside will have a valve open and it is rusted and stuck open. Never seen the rings rusted and locking up the piston in an aluminum bore engine. If you are sure the piston is stuck and not something locking up the flywheel remove the head a turn mower so the piston is up and pour in your favorite elixir from the web. Let it set for a couple days then get a chunk of wood and a hammer and hope the piston is not at the bottom of its stroke

That one of the problem here. We don't have the equipment numbers. It could a aluminum or a cast iron sleeved engine. But as mention carbon can jam an aluminum bore. Just recently had de-carbonize two Stihl brush cutters that have plated aluminum bores that where stuck because of carbon.