Aluminum when exposed to the elements forms aluminum oxide on the surface, similar to iron oxide, aka rust. The big difference is that, while iron rust will allow additional moisture that causes more rust, aluminum oxide "seals" the surface stopping (or slowing down) any more oxidation. That's the good news, if you clean the aluminum surface you will be OK, if you scratch the surface don't worry (a lot). Don't apply any paint or other sealer. Do pay attention to steel permanently attached to the bell housing.
That being said, one local restorer type took his aluminum bell housing to a steam cleaner business. He recommended that procedure to me, I didn't take mine yet and I don't know the cost. It did look good. A second restorer type used his portable home pressure washer on his bell housing and C-4 transmission and he recommended that method. (And, you can also use the pressure washer to clean your cement walks and driveway if you borrow one. Wow!)
Jim