seized engine?????

Now back to the grey oil. As I said, grey oil is an indication of water or coolant in the oil. Water in the oil is not good for an engine and if there is enough of it will take out bearings. If it is anti-freeze in the oil it is worse and will take out the bearings much quicker. Water and particlarly anti-freeze will not stick to the bearing surface and will prevent the oil also from sticking to the bearing surface hence a lack of lubrication and the bearings become tight (too brief and explanation but you have the idea). I think if it is seized due to this situation you would have noticed the engine labouring (running like it had a load) in the minute or two before it stopped.

In any case you need to get that old grey oil out of there and replace it with new before you try to run it again. I say you go about this in the order I have suggested because the starter situation is the easiest to check out first. I would suggest that if you get the engine running again that you then drain out about half of the new oil and replace that half with diesel fuel and run the engine a few minutes to flush it of all the crud that has developed from the grey oil. Then drain the oil and diesel mixture and replace again with new oil. You should then check your oil frequently for the next while to make sure it does not go grey again. If it does, there is probably coolant getting into the oil and that needs to be fixed.

If you really think it's seized, try to spin the engine over using a socket and breaker bar on the crank pulley bolt. On some small tractors, you can use a screwdriver and pry against the flywheel teeth. All depends of which one is easiest to get at.

The oil condition sounds like water in the oil, maybe just lack of regular oil changes, maybe something much more serious.