Keep original or replace with new? spanish chair Borge Mogensen  

Leave it
Looks good!, thats life as a chair.

If you try to replace the...
If you try to replace the leather on the back, it will never properly match the leather on the seat. Regardless, I agree that the aged/repaired leather give the chair character and add to its charm. I'd leave it the way it is. If you ever find yourself in a position where it breaks and or is no longer usable, I think you will have to replace the seat and backs.

Is it sound?
Can you use it, or is it weak and likely to break?
If it works fine (or until it does break) I'd leave it as-is. If it fails I'd replace both the seat and back at the same time. As already mentioned, it will never match. If they have to be replaced, let them acquire the same patina from use over time.

Thanks!
Seems obvious, however there is a tendency to over restore here! If it's not fully restored it is considered tatty! Lots of french polish on Danish classics! I feel it should be left alone as it functions well, so called experts are advising me to restore! Thanks for the input!