Your comment about the clutch would indicate to me to start there. Since you pull-started the tractor, we know it's not a pinion stuck engaged issue, so that limits it to the pony motor through pinion assembly. As trainz indicated above, removing the magneto and pulling the heads are very easy to do, so do that first. Of course for the heads, you'll need to drain the coolant but that's not normally difficult, and if the pistons can move, even a little, then they're "probably" ok, and you can refill the coolant. If the magneto turns over normally by hand, and the gear turns true/doesn't indicate a bent shaft, then you can set that aside as ok. If removing the mag does not free the pony up, then you can either refit it, or leave it off in case there is an issue with the timing/cam gear train.
But then that takes us to the pinion and clutch. Could you describe what exactly happens with the clutch? Does the lever "go too far" into engagement and beyond? If so, can you return the lever back to disengaged? Even if you can, you can pull the rear (well, front") cover off the pinion clutch assembly and take a look inside and study the mechanism while operating the clutch lever. If that's where the issue is, if you're lucky you might be able to poke around and move parts back into their correct place. If you can do that, then immediately adjust the clutch to confirm that it then operates normally, and that the pinion shaft can turn over ok. Also check for end float on the pinion shaft.
Then report back here so we can see where to go next. (which might be pulling out the pinion assembly)