If you go into the Music Symbols Editor and find the 8va symbol, you can delete the existing glyph (which is a single “8va” glyph), then add an “8” from the Octaves glyphs and separate “v” and “a” from the Octave Supplement glyphs. You can then adjust the size of the 8 to suit your needs. You’ll then potentially need to do a lot of faffing in Engraving Options > Octave Lines, and bear in mind that with this altered symbol, the settings won’t necessarily work the way they’re advertised. Pay particular attention to the setting for where the line meets the symbol, and whether it tucks under or not.