Pour some cold water in there and attack it with a stiff brush. That'll shift all that muck from the surface. Turn it upside down and let all the water and muck drain off.
When it has dried, stick the pan on a fire and melt the tar. Once melted, put small amounts into plastic bags and sell them as "Pitch". Lots of people want small amounts of pitch (but don't label it as Brewer's pitch, that's a completely different thing.)
Once your pan is empty, scrape out any residue as best you can then let the pan cool. Attack the remaining tar with a drill and wire brush attachment until it is looking cleaner. If you manage to get it really clean you can sell it separately.
Eric