Plugging the existing fitting is easy...
Drop by any plumbing supply and pick up a 5/8" OD threaded plug...wrap a little reflon tape around the threads and thread it in. Installing a new vent fitting is a bit more challenging...in fact, just about impossible if you don't have an inspection port, 'cuz you need access to the inside of the tank to do it. So the job may have to begin with retrofitting an inspection port kit...Beckson makes 'em...Ronco sells 'em. Assuming that you also want a larger diameter vent, buy a 1" "bulkhead" or "mushroom" thru-hull (NOT a "vent" fitting, a thru-hull)....the kind that's threaded and has a nut. Also buy some rubber gasket material. Using a hole saw, drill a hole in the tank that's a VERY tight fit for the fitting. Reach into the tank through the inspection port and insert the fitting in the new hole, threaded barb to the outside of the tank. Make a rubber gasket to go under the nut--it should also be a VERY tight fit....tighten the nut...and voila--new vent fitting. If you do enlarge the diameter, buy two bulkhead fittings...use the other one in the hull.
