Call Speedmaster. They list valve seats on their website, ask them which ones are used in their heads and have them send you some.
But I agree with N.T.- I wouldn't mess with them unless there's something wrong with them.
How do you replace an existing valve seat with one the same size? Seems to me I would have to go oversized with the new seat. Never going to get the bore in exactly the same location twice. But this is just me thinking - I've never done one.