Soak it down with high concentrtion Clorox bleach/water and then start the clean up, shovel would be the first tool!??
I have bought and cleaned up many a project, but the 64 Savoy I hauled home few months ago that has sat outside since 84 was by far the worse. I am glad my friend put me on it, but wow I have never seen such seat stuffing mixed with so much rat chit and piss ever!!!! It ate out the floors like acid. All fixed now!!!!!