Hi Larry and welcome!  I have fixed up a number of OMC pressure tanks.  I like them better than the newer single line tanks.  Anyway here is how I do them.  I tear them apart and clean and check everything.  There are two check valves one in the top part and one in the pickup tube.  The one in the tube is right down by the filter and it tends to stick.  Make sure that one rattles when you shake the tube.  I like to clean the inside with my shop vacuum.   The opening on top is big enough to get your hands in there.  If it is rusted bad inside that is a whole different story.  I paint mine with a good enamel paint and I use a roller.  Most rattle cans paints won’t hold up well when gas gets spilled on the tank.  People with spray equipment use acrylic enamel with hardener added.  That can be a big expense unless you have a body shop or a painter friend to help you out.  Gasket sets are available on ebay.  A seller name shabestari has reasonable decals.  They are not perfect too originals but they look great.

dale