Can I Breed Non-Galarian Pokemon?
Based on this guide, which I verified and confirmed, giving a Kantonian Farfetch'd an Everstone will allow breeding to maintain its regional form.
However, this will not work for Pokemon where the base evolution, such as Pikachu and Koffing, do not have a regional form. When evolved, their evolution would be the regional variant that is found in Galar region (Standard Raichu and Galarian Weezing respectively).
Another thing to consider that the guide says:
If a female Pokémon or Ditto holding an Everstone breeds, they will always retain their original form.