[Scripting Question] Need help with scripting a particular Pokemon

[CarneMorta]

Hello everyone! Recently I've been working on a pokemon fangame. In this game, there will be a pokemon that acts pretty much like Arceus/Silvally do. Depending on the item this pokemon holds, their form/second type changes, along with a special move they have.

The items they can hold are three:
- Fire Stone
- Leaf Stone
- Water Stone

How do I script/edit so that this Pokemon changes form based on what item they hold?

 

[Pokeminer20]

clone what arceus and silvally do. it's the ability that matters most, so just clone the data for Multitype into the new pokemon and rename the items and the species it's tied to