In a sense, both Gareth and Pete are both correct!
If it's only a 'commercial' consideration, then Gareth's comments are very valid.
However, a lot of us don't see these things as purely 'commercial' and will spend more than they are worth bringing them back to their glory days - it doesn't make sense, but there you go!
Personally, I would say that nearly any old BSA was worth the effort, time and money to bring them back, unless rescued from the bottom of a pond after many years or their condition really doesn't warrant it.
(indeed I would say that was probably the case with any other decent make of old rifle - even Wobblies!
) (just kidding, Wobblies are fine - just not as good as BSAs!
)
The Mk1 Mercury has a lot of fans and rightly so!
Anyone who doesn't have the time or inclination to do this sort of work on an old gun they are given would be well advised to pass it on to someone who would, but, at the end of the day, the enjoyment of bringing something made a long time ago back to nice condition, can far outweigh any cost in doing so - a labour of love, so to speak!
So not agreeing or disagreeing with anyone here, but what I would say is, if you haven't tried rescuing an oldie rifle - maybe it's something you should try once, even if it's not entirely a success, you will probably enjoy/learn from it and you can always pass it on to someone else if you come unstuck.
Old rifles deserve a 'second chance'!
Atb
ASM