I would think the Nuovo Record track BB (109mm axle) would be a good alternative if you don't have the original bb. Just off the top of my head, I think the modern Campag track BB is 110mm, but I'm not sure whether the taper's been changed or not.
Campagnolo taper hasn't changed, I've had vintage Campag Pista cranks on current sealed Campag Pista bb and on older loose ball bearing bb. As for the Mavic Pista crank, my LBS had the lovely fluted Mavic crankset on a Campag BB a while back no problem and good chainline.
As you say the difference is only that the bb length has changed. Modern ones are 111mm, whereas the vintage Pista cranks were 109mm. I had a 1980s Pista crank on the 111mm bb and the chain line is very good, but not quite perfect.
The perfect 109mm axles can be hard to come by, especially Pista ones, get expensive. I managed to get a NOS Victory bb for a double setup a lot cheaper, and these are also 109mm in length. I think people also tend to just measure the Victory one as 109mm and then sell it as a Pista BB! You can tell it's not as it doesn't have the 'P' or 'SP' in the code on the axle I don't think.
Hope that info helps in some way.