I don't know - I'd say anything sitting THAT long untouched is a special case that's probably not wise to do the soak and force method.
A lot depends on how it was stored of course - stored indoors in a dry climate is obviously a different case than outside in a wet climate.
But assuming somewhere in the middle - a lot can happen over thirty years. The cylinders can be so badly pitted they need to be bored anyways which means there's no sense continuing on this path.
Valves can be stuck solid so even if you get the pistons free you'll be bending pushrods, etc etc.
I think I'd start by pulling the breather tube cover and looking in to the bottom of the cylinders - then pull the head and see what you've got in there.
Head gaskets aint cheap for that engine - but I don't think you're going to be able to avoid ripping the engine apart to at least inspect everything.
It's inevitable - pull the head.
If you find things don't really look that bad -you can still keep applying penetrating oil to the pistons. And keep rocking the tractor by the back wheels.
That's how we freed ours up - put it in gear and just keep rocking back and forth. It seemed totally hopeless, but then we saw the flywheel jump a little. Then a little more... then finally free.