Why not scale to whatever your output format is going to be?
Shame it was letterbox and not squeeze … a lot of the resolution is already thrown away in letterboxing. And how does a letterboxed vid look on a widescreen?
The Beeb likes material recorded and edited in 16:9, and then put through an aspect ratio converter to generate a 14:9 master for broadcast.
On a standard 4:3 screen, the viewer then sees a thin black bar top and bottom, and on a widescreen a thin black bar left and right – unless the screen’s auto adjusatment kicks in …
So a compromise for you might be to “blow up” your letterboxed footage to 14:9, and similarly shrink your 4:3 footage to 14:9.
Of course if you want the band to look skinny, you might prefer to stay with the letterbox format, mask on black bars to match top and bottom on the 4:3 footage and reposition it “pan and scan” style.
Yours is going to be a bit of a makeshift repair job whatever, so it might be as well to make a feature of it, stretch the pix to make them look how you like, and do enough picture processing to make sure the low-res look seems like an intended feature and not a shortcoming!