Here is my idea after all the Hisuian Pokemon are officially revealed.
1. Stantler can be found in the same place as modern Sinnoh, and using Psyshield Bash 20 times evolves it into Wyrdeer
2. Hisuian Qwilfish can be traded from an NPC for a Johtonian one. Use Barb Barrage 20 times evolves it into Overqwil
3. Hisuian Sneasel can be traded from an NPC for a Johtonian one. Use a Razor Claw during the day evolves it into Sneasler
4. White-striped Basculin can be obtained from a sidequest that requires you to catch both variants of Basculin. Take 294 recoil damage to evolve it into Basculegion
5. Ursaluna can be found in the same place as modern Sinnoh, and give it a Peat Block during fullmoon evolves it into Ursaluna (Peat Block could be a rare item underground or obtain one from a sidequest)
6. Scyther can be found in the same place as modern Sinnoh, and give it a Black Augurite evolves it into Kleavor. (Black Augurite could be obtained via sidequest as well)
7. Enamorus should not be added until the post-game.
8. Other Hisuian forms are gotten via NPC trades.
9. Rowlet, Cyndaquil, and Oshawott could be obtained via a gift.
