Bear with me as a noob in these matters.
I have converted a book with the Japanese letters Ō and ō from mobi to epub and lrf with Calibre. Unfortunately the book shows up perfectly in Calibre, Firefox and Sigil - just not in the Sony Reader.

The letters show up as "?" in epub and as blanks in lrf. (I have the same problem with the name Žižek in a different book.)
Is there a simple way to handle this problem that is available for me as a noob? I do not like to flash the firmware or to learn programming just for this book. Usually I would try to avoid these issues by converting it to pdf, unfortunately as the book also contains maps the results of converting it with Calibre to pdf are somewhat unsatisfactory.
I always understood that lrf-files also include required fonts. Why doesn't it work with this file?
Thanks in advance for your help.