If that pump is just rusty and still working, I would knock off the rust and paint it with Epoxy Enamel paint.
I agree-- why replace a working pump? If the rust bothers you, I would brush on POR-15 Rust Preventive Paint as a primer under any top coat. http://www.por15.com/POR-15-_p_8.html It can also be used without a top coat. It becomes a dull dark grey to black if there is no top coating, but it does cover and then inhibit rust.