Editing default (Vanilla) code from MCreator

For the purpose of my mod (I'm trying to create a mod that adds things that are only in Bedrock version to Java, such as Baby Villagers with bigger heads, or sheep with full colors when dyed) I need to be able to edit the code of Vanilla Minecraft, but I do not know how to do that.

Also, I got my hands on a version of Techne, so I can make models and implement them to MCreator, but I wouldn't know how to edit a creature that is already in Minecraft without adding a completely new mob. 

So I need help with those two problems. Any help would be amazing!