IMO, freshly draining oil (yes, radials DO leak that much!
) would not likely run that far aft on the belly, but would find a lateral panel seam and drop to the ground closer to mid chord-ish. After takeoff on a subsequent flight, that oil would become spread over the belly adding to the overall "grunginess" so prevalent there.
Based on personal observations of SPADs in a prior life, I would "grunge up" the belly some more, and add some brown staining to the drop tank from previous shutdowns... (there are many pics online)
Overall a convincing effect and a beautiful model!
After decades of study, it has become obvious that Murphy was an optimist...