If it's worth it to you... I'm at the pint where I see prices as a base. If a particual item is worth "by the book" $100 and it's priced at $110 but I really want it, I'll but it. The gun is only worth as much as you're willing to pay for it - or how bad you want it.
Ok that being said, if it's to be a working gun why worry about the bullseye? The game animals won't care nor will having the correct swivels matter to them. Buttplate... easily replaced for $20 or less. For an investment piece, it's proberly not such a deal since after the money is spent bringing it back to original specs, the additional funds won't likely be recovered from selling it.