Welcome to CHR!

Not sure exactly what you're looking to do. By the discolored areas do you mean directly under the carb where the manifold bolts are? Is your intent to then repaint the manifold? Is the manifold off the engine? If so, go after the discolored area with a wire brush in a drill motor as you can't hurt the cast iron.

As to the choke - yes you can, but it requires a kit to do such that will set you back $75 which may be better spent on a new carb rather than a 40 year old unit.

Snap a picture or two and give us some more information on what your goals are.

Again, welcome to CHR.
Glenn