I would still back up a little and make sure it is the pistons that are stuck.If you put oil on them,and stored it so moisture couldn't enter,it shouldn't have stuck.If you put a battery in it,and tried to turn it over the starter drive can lock up on the flywheel.You will swear the engine is locked.I ask because it sounds like you haven't got the head off yet,and if it isn't off how could you tell they are stuck?I see this quite a lot where folks have an old car or tractor sitting for years.They first put a charger on it,with this wet dream that the battery might come to life.Then they try to turn it over.The old battery and charger put enough juice through it to just kick in the drive,and there it sits,locked up.I just looked at a Farmall H that had a locked up engine after sitting all winter.They towed it.They put crowbars on the crank trying to free it.They decided to have me do an in-chassis overhaul on it.I told them to hold up,and I went out to look at it.I unbolted the starter,hit it a couple of times with a hammer and heard a clunk as the bendix retracted.I bolted it back up and spun the crank by hand.It must have shot a half gallon of mystery oil out of the plug holes and all over their garage wall.They had charged the battery overnight and hit the starter,but there wasn't enough juice to do any more than lock it in.