Branched Alolan Evolution
Source: https://www.pokecommunity.com/threads/branched-alolan-evolution.430692/
Back in Feb 2020, AsparagusEduardo created code to implement a way to be able to evolve Pikachu, Exeggcute and Cubone into both their regular and Alolan Evolutions.
By holding the Strange Souvenir, and doing the evolution method in USUM (eg. Thunderstone) they will evolve into their Alolan evolutions.
This code adds the item Strange Souvenir as well as the evolution method.
To add the Alolan forms, use this guide as reference: How to add a new Pokémon species
You can pull from the branch: https://github.com/AsparagusEduardo/pokeemerald/tree/AlolanEvolution
git remote add AsparagusEdu https://github.com/AsparagusEduardo/pokeemerald
git pull AsparagusEdu AlolanEvolution
Note: if you have already added other evolution methods in your decomp, adjust and increase the numbers on the #define evolutions lines in include/constants/pokemon.h
#define EVO_ITEM_ALOLA 0x001F // specified item is used on Pokémon while holding a Strange Souvenir
#define EVO_LEVEL_NIGHT_ALOLA 0x0020 // Pokémon reaches the specified level, is night, holding Strange Souvenir