How do i make this rideable kart functionable?

[Makego]

I wanted to transform this kart from Shocker's Graphic's Pack 9 into a rideable yoshi (npc-228), i was easily able to alter the yoshi sprite into it, but not the "riding yoshi" one.
For the head i put a blank png file and it worked as intended, but i wasn't able to resprite the body, even putting a yoshib-8.txt into the level folder and changing its size and number of frames.
What do i do?