It is entirely possible to buy a new top screen: http://www.modchipstore.com/NDS-DS-LITE ... 16366.html

I bet if you searched around you could find one for less.

Although... with all the cost of parts (shell + screen) you're looking at a substantial chunk of the original price, so it might be best just to suck it up and buy a new DS...

Good luck though. If you do end up trying to repair it I might remember some pointers for the actual disassembly that would come in handy.