You’re right, it’s not exactly the procedure:
1. goto https://github.com/rism-digital/midi-player/tree/master/instruments
2. get both .data and .js file of the instrument you’d like to play (for the .js, download the .js.gz then uncompress)
3. rename the .js file to wildwebmidi.js
4. copy wildwebmidi.js manually in Music Sheet Viewer plugin directory (under music-sheet-viewer/js), instead of the current one (piano by default)
5. copy the .data file in the same location
6. remove existing wildwebmidi.data (the one for piano)
Remember to put aside the originial wildwebmidi files if you want to revert to piano, otherwise you’ll have to reinstall the plugin.
I played 005_electric_piano_2_chorused_yamaha_dx with that method…
Good luck.
Etienne