[Arran]

Joseph;
I would give that bottom cover a soak in a mix of vinegar and water, that crusty rust will be gone in a few days. One method I figured out was rather then immersing the object in a container full of the vinegar/water mix you can also wet some wads of rag or paper towels and place them on the crust which works well on top of a chassis where you don't want to dismantle everything. Hopefully using the chassis as a washroom was the worst of what they did and that they didn't naw on any coils.
Regards
Arran