1) Refer to this post:
2) Pokémon from gens prior to 5 will have Poke Transporter as the location.
3) Just recreate the Pokémon. All that is preserved from gen1/2 to 7 is the species, nickname, OT, trainer ID, gender, current level, and if it was shiny.
I’m not sure if it’s in PKHeX (I reckon it’s there); I remember seeing it on the old old save tool pokesav. If you know the offsets could just make a plugin or something.
You're in luck I was fixing my Silver save today and had to fix my map location. Here you go just rename the file and you should be good to go!
Pokemon - Gold Version (USA, Europe) (SGB Enhanced).Sav