You will need an old Mac that can run the app and export. With any luck, you still have that Mac, right? Yea... A G3 is your best bet since it can run OS 8 or 9. The various builds of Mosaic were twitchy about which versions of the OS they would work. IIRC, Composer’s Mosaic was required for OS 9 and stability was a major issue.
Macintosh Garden is an excellent source for old builds of the Mac OS and various builds of Mosaic.