Does the crystal fit directly into the case?
If so then it is possible to fit by slightly expanding the case :)
Last time I did that was with a pocket watch, with a similar “knife edged” crystal.
Get an old skool 60w light bulb and a pair of leather gloves - garden ones will do.
Put the case on the light bulb and turn on.
Leave for a while, can’t remember how long, but 5 or 10 minutes should do.
Remove from bulb with gloves and place glass into now larger hole.
As it cools it will shrink and hold the crystal securely assuming the correct size.
Dave