Welcome Hugh, we haven't met but I see you've been here a while.
A properly repaired headstock wouldn't stop me from buying a vintage Alembic that I really liked. How much got discounted on the sale would depend on how much the repair showed. Alembic having done it, It could be indiscernable. It could still be obvious, depending on how involved the repair was. Me, I wouldn't pay 'full price' (whatever that is...) but I wouldn't expect a half-off sale either. A solid repair is a non-issue from a structural standpoint. So is the rest of the bass in collector's grade condition and this is the only wart? If so, that's a shame. If it's just one more scar, congratulations... you got yourself a great bass that you don't have to worry about getting a scuff or two on.
True story; just happened this past week in my shop. A regular client of mine bought a Guild 12-string online. When it arrived, the headstock was broken almost completely off, right behind the nut. It was shipped, tuned-up fully to pitch, and the headstock was unrestrained. I'd guess offhand it got dropped and the guitar got 'whiplash' inside the case... annnnd... snap-ola. The only thing holding it when he got it was the truss-rod cover and a little bit of the veneer face. The three-piece neck was snapped clean. So it landed on my bench with a great big question mark, and the conversation went like this... Yeah sure, I can fix it, and it'll be a solid repair, and you can play this guitar forever. But what I can't do is make it invisible. I also can't give you amnesia, so if it's going to bother you, after I fix it, sell it to someone who won't care and buy another. There's plenty more out there. I just strung it back up this morning. We'll see how they feel about it tomorrow. (it sounds amazing... I'd keep it)
So anyway, that's my advice... if that headstock repair is already bugging you, pass on this one. There's plenty of vintage Alembics out there without issues. Good luck.
