a) I have had no luck with any attempts to clean factory gooey moldy dirty gaskets. They have a Very strange composition and accompanying resin-sealer. The do a great job but wow what a mess. None of the usual methods have worked for me, usually the side to the sheet metal housing retains the fresh white color, the side towards the lens back is nasty, moldy, dirt impregnated, deformed, dried, gooey or over sprayed during poor masking.
b) Be careful when sourcing reproductions. I have found various sources actually supply standard foam gasket die cut and thin, too thin to allow for proper compression upon the lens being tightened down, the shoulder screw bottoms out and the lens will still rattle around.