One simple solution might be to simply make the wheels removable. Then all you have to do is lift the tractor a few inches and slide them on when you need to move it.
You can also make some sort of hinged board flap with wheels. It would sit vertically along the back side of the tractor when not being used and hinges horizontally to fit under the tractor when you need to move it (would require you to lift the tractor up and swing the hinged boards under the tractor).
Lastly, if you do mount the wheels so the tractor sits flush to the ground, you can still roll the tractor if you tip the other end up high enough (sorta like a hand truck or wheeled garbage can)