You find the crystal based on the case reference, not the movement caliber. That's the same with the crown and the stem. Do you have that number? It should be something like 166.050.
£142 is a crazy amount for the original crystal in my opinion. I don't know much about cosmics, but it makes sense to me how it would be hard to find new crystals for them. Because these models are front-loaded, the only way to access the movement is by removing the crystal. People easily mess up the crystal while doing that, which I have seen with many cosmic models sold on eBay in the past. You can tell by the cracks around the crystal.
How are you thinking of replacing the crystal? You will need a crystal lift tool like the Bergeon 6400.
If the stem issue you described is with the cosmic, that one should have a split stem. It's hard to tell without seeing (and I'm not an expert either), but the issue you described does not necessarily mean that you need a new crown and a stem. It could be the connection point of the two parts of the stem, but I'm just speculating here. You could reuse your current crown with a new stem, if that's where the issue is.
You might want to take the watch to a watchmaker to have them figure it out for you. If they have an Omega parts account, they might be able to get the crystal at a more reasonable price from Omega as well.
