That should be a common part to find in the USA . Many aperture sizes & outside diameters to choose from . Gun parts dealers at a local gun show should be able to help you out . Search Lyman 48m on EBAY & see if you can find them . I doubt there is only one specific size for the fm23 . That would be an option for the shooter .
The 48M on my fm23/36 has a 2.5m/m ( .098" ) aperture .
I have a 48M with a Crown marking & a 1.2m/m ( .047" ) aperture .