Have you tried contacting WC, plus a bunch of the companies who make through
hull mushrooms? They'd be my first stop. And maybe someplace like Davey & Co. who carry & make all sorts of old
school boat hardware. Also try some of the nautical foundrys, like the one in Port Townsend, WA
Can't imagine that any good sized boatyard doesn't have such tools on hand when you think about it.
That said, I should think that it wouldn't be that tough to
DIY one. Get/machine a cylinder of the proper diameter to fit the hole in the through hull were it fully round. And then machine out notches which will grip the 2 ribs on the inside of the through hull mushroom.
You could even cast one using bog (epoxy, & structural filler) to use to take measurements off of it when machining it. And when you're machining it, square off the end which stays on the outside so that you can put a wrench on it.
Perhaps the last ditch solution is to cut off the mushroom
head as close to flush with the hull as you can. And then once you unbolt the seacock, the remaining piece of the through hull should come out via your unscrewing it & the seacock from inside of the
boat.
That, or cut it off where it mates up with the seacock, & then cut it into pieces with a hacksaw blade down it's "throat".