The backbox part of my exhaust is heavily corroded and on its way out. My car is a 1.4 16v. Before I just replace it with a stock part, are there any other options I could consider which might improve the car's performance and/or sound? (I don't want it to sound like a fighter jet, mind you)

you can buy after market backboxes, like the vrs. best bet is to visit alba tires if there is one near u and they have lots on display and can even fabricate one to ur needs

Found this...

Sebring / Back Boxes / Skoda Fabia / Euro Performance Car Parts

Bit expensive just for a back box tho.

look up Essex Racing..

Universal backboxes from £35

I did it for you... Performance Exhausts: Essex Racing: car accessories, tuning, parts, styling