watch some videos of wreckers / roll backs on youtube. kinda surprising some companies showing off, how fast they can come in and reposes a vehicle in a couple minutes. with correct equipment. a tractor is easy pickings.
unlike cars/trucks with hoods. tractors generally are wide open for all the fuses, wires, brakes, etc... and on older tractors, 1 key works for all make/model of a given tractor or a whole bunch of models from a given manufacture.
steering wheel locks, wheel locks that work on a car/truck = nada for a tractor. to much room to turn the steering wheel, and tractor most likely laugh at a wheel lock and roll right on over it.
move a lever/joystick and hydraulics release pressure and let things move under there own weight.
unlike construction equipment, were hoods / side panels might have a "welded on" hole were you could toss a paddlock to keep folks from getting into and around the engine... regular agriculture tractors... to smaller cuts / scuts.... are more likely all fiberglass / plastic or like, and would damage the panels / hood if a lock was installed, to keep folks from getting to say the fuel, or turn on a fuel valve or like.
============
looking at tractordata for your tractor...
TractorData.com John Deere 5093E tractor information
hoping in it, and just driving off could be a concern, vs someone backing up a trailer and taking off with it.
==============
would be nice to have some sort of "kids toy" that as it moves about generates a small bit of electricity to make lights flash and like. using something like that make it water proof, and toss it into a tractor tire, before the final bead is seat. little GPS sensor or something... would more likely be a ticket for tractors. or some way stickied or bunge strapped to the rim, before the tire is put onto the rim. heck general wifi access for internet access may even work as much as you can drive around and pickup wireless access from someones router.
===============
google "car gps theft" looks like a wide range of cheaper units, that you might be able to hide up inside a dash (around steering wheel or a side panel gauge area), and have it tangled up in some wires, so not easy to find / see.