Oh wow! That looks amazing.

I'm not sure about collectable, but it's certainly rare - particularly with so much of the original kit in place. The Allvit rear mech was a normal upgrade during the 70's for older bikes with the under-chainstay bracket derailleurs when replacements and parts for the latter became hard to obtain.

Up to you what you do with it. I'd just change anything necessary to make it a good ride - then find a stoker and ride it! Unfortunately my intended and very keen stoker (my youngest son) has now moved away and I can't find anyone else willing to rid on the back of my tandem, which is far below the calibre of yours.