Most inserts clip in to the bezel ring and it’s easier to push them out from the rear side. So I would take the bezel off. (Watch out for the spring obviously)
I would then measure the insert with callipers and then try the usual suspects to see if they have something that fits. (Yobokies, dagaz etc)
I have had limited success in fettling and insert to fit but it is never a perfect job.
Trying to find inserts has stumped me more than once. I am currently looking for a scuba pro 500 insert. They are like unicorn poop