I replaced the back and front of an Iphone4 on the weekend with a screen (and LCD) and steel back both off ebay, came with the tools n’ all.
Its a (very big) bit fiddly, just make sure you get the one with the “screw mat” or its a PITA, I didn’t and miraculously ended up with a few ‘extra’ after rebuild! still it works and its not that hard to do.
total cost £40.00 and some colourful language.
check ebay and see if replacement parts are available?