The stemlength of the Longa cannot be changed.

% The stemlength of the longa cannot be changed. It is too short for the
Baroque NoteHead style. Longas are normally used as the final note in
Renaissance music.

\version "2.26.0"

\relative c' { \time 9/2 c2 c\longa \override Stem.length = #2 c2 c\longa
                \override NoteHead.style = #'baroque c2 c\longa \override
Stem.length = #6 c2 c\longa }