Optel watches are great but I do sometimes wonder if they sealed the displays with bubblegum.
I'm afraid that there is nothing you can do to repair a display which has sprung a leak - it's got to be replaced by a working one. Unless someones got a box full of spares somewhere then it's off to ebay to hunt after a beaten up working watch I'm afraid.
Unfortunately, later none stem-set Optel displays (whilst being the same size and having the same digits) have the contacts in a different place and so aren't compatible with segtronics etc.
If you do get a replacement display then it's easy to swap. Take the back off the case. loosen the 3 screws holding the module in place and remove it from the case back. Then you just gently slide the metal clip holding the display in place from right to left until it comes off. The display and polarizer can now be taken out and replaced.