Yes to all your possibilities - anything driven by the engine could be stuck enabling only the amount of lash you're seeing. A stuck piston could allow allow that much lash depending on where in the stroke the piston is stuck - less at half stroke, more at full stroke (either top or bottom). I'm not familiar with this tractor but if you're able to remove the water pump and the generator, it'll only cost you gaskets and then you can positively confirm it's not them. For the camshaft, if you can see any movement at all in the rockers, that would mean that the camshaft is not seized but there could still be something preventing it turning further. Siimilarly the oil pump.
But if I guessed, I would say a piston since you had detritus in the manifolds, there's every change that there's also some in the cylinder(s) and the rings have stuck. Again, you'd be out gaskets but if you pull the heads, then you could verify positively. I think there's a couple of ways you could start to analyze but I'd start with the engine side covers because they're really easy to remove, then go for the water pump / generator if they're reasonably accessible, then I'd go for the cylinder heads. Everything is just gaskets up to that point.