Before you think about moving that, you'd be well advised to slap some good diagonal bracing on the run portion. That kind of construction is NOT going to take well to being moved and jostled. It will work its way apart fairly quickly, even if you've used L-brace hardware. Use 1x4 or 2x4 lumber, screwed well to the frame, in large diagonals both directions on all sides.
To add wheels, there are many options, but most people use lawnmower wheels on appropriate-sized bolts as axles (must be put through significant sized lumber on tractor frame, which may mean adding lumber).
Are you suuuuuure that thing is going to be light and strong enough to move more than once or twice, though?
Good luck, ahve fun,
Pat