A good while ago I used these guys to repair the rubber buttons on my 550 unit. One of the buttons had gone completely meaning the device was no longer waterproof, and a replacement case was hideous money. They did a top notch job and were very quick with replies to my questions etc. I would definitely recommend them and they do a range of fixes/repairs for the 550. They are US based which means a longer turnaround time, but worth the wait IMO.
If it's just the top surface it's about £11 off eBay for a new one. Might need a little bit of soldering.
Have a search, because I'm sure you used to be able to just buy the whole screen together, which iirc is just a case of refitted and lifting a little connector to slide in the connector and put everything back together. I did a couple a few years ago, and only took about 15- 20 minutes all in.
Something rather odd has happened, whilst looking around through drawers I have found a brand new and unused 550 in a case. Never been used. I'll still repair the old one, but it is a bit less urgent.