Hi @Chris_Davey,
apparently, you also want to substitute the default Longa notehead with the Breve notehead and have posted this solution:
I think an easier way would be not to scale the normal-size notehead up to 120 %, but to use the original larger noteheads. They are to be found in the optional glyphs range of the SMuFL specification:
U+F4BA: noteheadDoubleWholeOversized
U+F4BB: noteheadDoubleWholeSquareOversized
U+F4BC: noteheadWholeOversized
U+F4BD: noteheadHalfOversized
U+F4BE: noteheadBlackOversized
See this topic on the Musescore website which I have found on the Dorico forum in this post.
These larger (“oversized”) noteheads are listed as “Recommended stylistic alternates” in the SMuFL specification (see https://w3c.github.io/smufl/latest/tables/noteheads.html#recommended-stylistic-alternates. However, I have not found a place on the SMuFL github where their above-mentioned Unicode position is specified.