I got my pikachu in Johto, if I were to evolve it in Alola would it become an alolan raichu or be a normal raichu because it isn’t originally from Alola?

Unfortunately, should you be hoping for a psychic raichu, the truth is that Alolan raichu are the byproducts of adaptive evolution. That is to say, the pikachu of Alola simply have a slightly different genetic code than Johtonian pikachu (or, for that matter, any other), so pikachu brought to Alola will always evolve into the non-psychic variation. To that same end, pikachu brought out of Alola to other regions will always evolve into the psychic variation.

Thus, to answer your question, a pikachu born in Johto will only evolve into non-psychic raichu.