RPG Collision mask basics - I'm new at GML, please help



Hey. So I'm trying to create this rpg where the main character is riding on a motorcycle using the built-in physics system, however, the collision mask doesn't change when I change into sprites that are different from the sprite the collision mask fits into. I understand why this is happening, and I understand it has something to do with creating a invisible mask, but I have been trying for days and I just can't fix this bug. I'm really new to GML (enough to fix a simple sprite bug using some simple code, but not good enough to look at complicated script and not be able to tell the difference from ancient Egyptian hieroglyphics) so I can use all the help I can get. If you need a basic summary of where I am, I'm at Heart beast's episode 9 of How to make a RPG. Thank you, any honest help is appreciated.