Gentlemen, thank you for participating in this valuable discussion.
Unless my memory betrays me, I remember seeing partial koshirae with papers. I bring this up, because I think it is important to preserve original koshirae, even if parts are missing.
Is it acceptable to add a tsuba or kozuka/kogatana to such a koshirae? I think yes, as long as it is disclosed and can be reversed without harming the original koshirae.
I too, am not a big fan of fresh tsuka make. But, if the original wrap is gone by way of the 'menuki oni', then why not?
I must say, I am more on the side of preservation first. However, playing devils advocate, maybe things like re-lacquering a busted saya, could be considered as maintenance, in much the same way polishing a blade is.