Custom Items for Cobblemon

I want to create a data pack for Cobblemon that allows mega evolution. My general experience includes coding in rust, java, C# etc. Datapacks are new to me but I am kinda limited to using datapacks. 


    

  1. ) To create custom items do I need to overwrite an existing item?
    2. 

  2. ) Is it possible to create custom GUI's in and for Datapacks?
    3.
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