@porcupine59 lf you haven't already done so, you might want to download the official
Sigil User Guide.
Then read the
Include Custom Fonts tutorial. If your book was properly encoded, the
Junicode font should work.
You probably only need to embed the usual 4 default fonts.
Junicode
-Regular.otf
Junicode
-Bold.otf
Junicode
-Italic.otf
Junicode
-BoldItalic.otf