Relic Castle

I know that the default overworld sprites that come with Essentials are from FRLG, but the battle graphics, pc, party and summery screen are all more gen 4ish. I want to make a game that's more gen 3ish, with gen 3 sprites and such. I've tried looking for resource packs or ways to edit these things myself, but I haven't found anything that's actually finished or helpful. Do you guys have any advice?