Mine was in way worse condition than yours.
Start by sealing the speedo hole.
Oven cleaner,wire brush,putty knife,tooth brush,scotchbrite,hit it with a high pressure hose. Repeat if necessary.
Next put it where your going to paint it (preferably standing up) and hit it with brake cleaner so it runs from the top to the bottom.Let it dry. Now its clean.
Paint it with your favorite spray. This one is with Cast Blast.
Do you plan on rebuilding or putting new seals and gaskets in ?
There will be a little water inside. I would at least open the side case to remove the water.
For parts....Wayne at Brewers.