With any rust I would check checking for clogged filter screens. One the fuel pickup tube and if you are lucky one on the carburetor tower in the tank reservoir cup.
As the idle screw the IPL does shows it to a fixed jet. And as Mowerfreak mention the fuel pump diaphragm does usually need replacing.
As for rpms problem it might be that someone installed the wrong governor spring. Note there two listed on the IPL and they don't say what the differences are.
