My seacock was also a pain to get out. Had to get a flush cut saw to file away the old bedding compound (4200 or 5200) then some loving taps with a hammer and a torch to break the first quarter turn. After it came off without too much more elbow
As for the Spartan versus WC, Spartan has the name on the handle. Check your other seacocks to tell.
Spartan has redesigned their seacock slightly from my 35 year old original. Not sure if the plug shape or size changed as I didn't try. I am not sure how you could test whether it works before splashing. Finding out after would be expensive (another hull and you are in the same position).
I suppose a good machine shop should be able to do the treads and welding. Just make sure they can duplicate the jam nut. This is assuming it is even possible to weld old bronze. If you really don't want to take off the old body, I'd give this option a look before trying a new plug. At least the chat with the shop is free.
While you are down there, you are also greasing and lapping, right? Spartan sells the necessary materials.
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