Nice find! I'm not sure how to go about replacing the screen or with what, plus I'd be afraid of breaking something. Since you only spent $20 there's a couple of options open and you'd still come out way ahead of the original $500 asking price for that radio:
#1: Purchase a copy of any compatible scanner software such as Arc796 or Proscan, and use computer control to see the display on your computer screen (menu, 5,1 I believe will activate PC control - I can verify that at home tonight if you like). You can get just about any software you want for around $60 or so, and most if not all come with a demo version you can try out before you buy.
#2. Ship it off to Uniden and let them fix it. They'll also be able to give it a once over and fix any other issues it may have. It might cost a bit more than the software solution but when they're done with it, it'll be like a new scanner.
If you're really brave and up to the task of replacing the LCD, take it out and see if there's a model # or something on it, then google for that exact part number and see if Mouser or someone like that has one they will sell you. I'd probably spend the extra money on getting Uniden to fix it though, simply because at $20 you basically got that radio for free. Even if it was a complete dud, that P25 card in it will fetch $250 on ebay any day of the week. You came out way ahead on that deal. Why can't *I* ever find a pawn shop like that?! All the ones out here are way too internet savvy to price a digital scanner that low.
-AZ