If you know the name of manufacturer they have a "cut away" view of the valve. There are several "low risk" ways to allow freeing up of the internal valve parts without significant leaks.
On my type, there are two "packing glands" and if they weren't so cheap, I would rebuild the internals for about half price of a new one. One gland is at the valve stem and the other on the big side of the valve body.
Doing what Seadaddler said in his post#6 would allow easy replacement while afloat.
BTW his plunger idea also works to test a new thru hull while on the hard. Just screw in a garden hose fitting to where the handle went, after making a 3/8th inch hole in that red plunger head. Get the water flowing at just the "cracked position" and press the plunger against the boat and observe the new thru hull inside.
Jim...