Where did you find the display case?
Got it from my local bike shop. They can get display cases from the same place they get their Garmins from.
Contact Garmin first and see if they will warranty it for you first.
I went this route because Garmin quoted me 90usd to "fix" my broken tab. I can pick up a new 200 for 100.00.
The process is fairly simple. Remove the torx (#5) screws, open case, the screen and main board will lift out.
Battery is taped in...so be careful when lifting it out. Trickiest part is removing the thin brass speaker. It has some silicone around it. I used a really thin and small screwdriver (jewelers type) to gently scrape the silicone out. After that...I used the same thin screwdriver to very gently pry up the speaker
Once the speaker is out...you'll notice it has some adhesive on it. It will still be tacky enough to stay secure in the new case. Use a small amount of clear silicone around the speaker. I assume the silicone is there to keep moisture and dust out.
When putting the new case back together...make sure you take the rubber gasket from the old unit and use it on the new one. Display cases don't come with one.
Finally I peeled the serial number off the old unit and stuck it on the new case.