Just watch out for the spring, it's like 4 inches long and a half inch across, it sits in the piston so it shouldn't come sproinging out, you'll feel the tension as you loosen the screws. I use a magnetic screw driver so I don't lose the screws, and then a bigger magnet to retrieve the screws I lose.
Ya done good lookin' at the diagram of the diaphragm, it's easy, don't be skeered. Slide the piston up, examine and replace it with the needle back down in the main jet.
The red goop is indeed gasket maker, the rubbery kind, called RTV, or BVD, or LSD, sumpin' like that, it says not for fuel use, but it held up pretty well for a while.