A helpful source regarding this (plus potentially other lost media)
From The Lost Media Wiki
I recently found this website called characterdesignreferences.com which archives various concept arts and designs from tons of different animated movies and TV shows. https://characterdesignreferences.com/ This source could be very useful in the future.
I searched up The Land Before Time on there and there was a lot of concept, storyboard sequences, cells, etc, listed there. Including the "Littlefoot and the Snake" scene. https://characterdesignreferences.com/art-of-animation-2/the-land-before-time?rq=land%20before%20time