Regardless of the material, once you find them, I'd check to be sure all the sea cocks actually work. On the 1987 Moodly 376 I recently purchased the raw water sea cock for the engine did not close. The valve handle turned, but the mechanism inside stayed open. This was quite a surprise when I went to clean out the strainer during our first sail. Fortunately, the other other sea cocks all seem fine.
There is 1 in engine compartment next to prop shaft. 1 in heads cupboard (on waterline) , 1 under sink ( on waterline), 1 under the seat next to the chart table - lift the wooden panel up and the rest are in the cockpit locker - you may need to lift the false floor up.
I guess until you inspect them you will have no idea what they are made of as they may have been changed to anything by previous owners.
Good luck - we love our 346....she has handled f9 in the channel and is very comfortable.
Well worth joining MOA if you buy one.....I saved our £20 subscription in the first day
Any more questions or just want a chat then feel free to give me a ring 07968050167