in version 1.9 it is easy;
1. remove the percussion Preset from your sf2
2. in Tools> Global> remove unused elements
3. open your new percussion sf2 and draw the percussion Preset into your soundfont
4. save it under a different name
regards
With the “new percussion sf2” you mean the other soundfont (Super_Mario_64_w-sq.sf2) right?. Oh and a fun fact: i want to use the percussion from that sf2 cuz its the same percussion within the game, unlike pablo corners ome