I clean my aircraft undersides with Go-Jo (the original, NOT the pumice version). Wipe it on by hand or rag, wipe off with a rag. It removes oil, grease, and really hard to remove avgas (lead) stains like magic. Haven't tried it on the tractor . . . never thought to.
If you consider WD-40 is a good solvent (and it is for some things), it's mostly light oil so you can save money over WD-40 by using kerosene, paint thinner, Stodard solvent, or oderless mineral spirits.