[Arran]

I would remove the dial scale and wash that whole chassis with a solution of about two tablespoons of TSP in a gallon of hot water. The solution will clean all of that dirt and oxidation off the chassis and the aluminum cans, though sometimes removing the cans makes it easier to clean the chassis. I pour just enough of the solution over the chassis to cover it and start scrubbing in every nook and cranny with a toothbrush, if that isn't enough I pour some more over it. After the solution has been on there and worked around for five to ten minutes I then rinse off with liberal amounts of hot water, then I place the chassis in a warm place to dry with lots of air circulation and leave it there for several days.
I've explained this approach to people several times but they are often horrified, but water is only really a problem with electrical equipment if it's wet and is energized, or it stays wet or damp for an extended period of time. The funny part was one of the guys who was horrified by my use of TSP and water was someone who advocated using brake cleaner, if you want to talk about aggressive solvents that stuff is up there with acetone. The one thing I would be careful about is not to submerge the power transformer, but this is more like a shower rather then a bath, and on this chassis it looks like it would be difficult to get any water in the windings by accident.
Regards
Arran