Good question. The original Mk7 tenor neck was the first Selmer experiment into the 'high-arch' design which places the cork end closer to the sax body, changing how the sax hangs on you along with the positions of the arms and hands. It is still being done with the Series II. Selmer finally returned to the late MK VI neck design with the Series III. The Ref 54 neck is in the early MK VI profile which many do not like on real MK VIs and probably Mk7s, although I actually haven't heard of this combination. It could very well play great but I doubt it.
However, the Ref 54 neck has its own value and you should at least be able to trade it for a good Mk7 neck or sell it, so I'm thinking there is a minor deduction for not having the correct neck. Usually it would be a big reduction in value, like on a MK VI in the range that had a numbered neck, but neck-numbering stopped around #140,000, well before the introduction of the Mk7, so any Mk7 tenor neck in equivalent condition to the sax should be okay, along with the SA80 Series II neck.
I would first want to make sure the Ref 54 neck needed to be replaced - again, it could be a great combination. Stranger things have happened. Some Mk7 players have used MK VI necks too, so the Series III neck is definitely worth a try.
The last factor is condition of the sax. If its a 'mint' condition sax, looking like new, it would be a bigger reduction without the original neck because the sax is correspondingly going to cost more because of its condition. A well-used sax I think would be less of a reduction without the original neck. Keep in mind that all these estimates of mine are based on the sax being sold with a working Selmer Paris neck in good condition. Not having a neck at all would be a large reduction in value.
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