That will be a larger scale project than working on a riding mower.
I would expect to have to disassemble the engine if it doesn't turn over, maybe you get lucky and there's only water in a cylinder, and you can clean up the rust and get it going, but more likely something's not working right inside. And to do that, you need to split the tractor, as the engine is being used as the front half of the frame (so you need some larger lifting equipment than mowers), as well as a way to move a pretty heavy object with two wheels on one end. And, depending on what they've tried to get the engine turning, it may be they've damaged the engine just doing that (you can wreck the starter, flywheel, crankshaft, piston/rod).
Another thing to concern yourself with is the injection system, it may not be working properly, and can be expensive to refurbish or replace the parts for it.
And you can't really test anything without the engine running, either the transmission or the hydraulic system.
You might get very lucky, and it's some trivial thing to get it working again, but IMHO it's not something you'll have running soon without working on it full-time and putting a fair amount of money into it (and assuming you can get the parts and any work done externally that you can't do yourself, like maybe the injection pump or injectors need rebuilding).