Thank you all for the suggestions and comments. I decided to take a literal approach and preserve the distinction between the preterite and the imperfect. I have found a couple of examples that corroborate my choice:
"I'm translating a book, which involves logic and quoted the sentence from Lewis Carroll's Through the Looking Glass: "If
it was so, it might be; and if it were so, it would be; but as it isn't, it ain't. That's logic.
https://english.stackexchange.com/questions/210451/if-it-was...THE ADICTS - "And
it was So!" (title of a song)
https://www.youtube.com/watch?v=SMnUin48PKs"No.
It used to be so but not anymore."
https://forums.obsidian.net/topic/111048-disintegration-and-..."I know
it USED to be so, but I've missed a lot of quieter rules changes over the years."
https://www.reddit.com/r/mtgrules/comments/w834px/if_somethi... (emphasis added to all quotes)
By taking a literal approach, I was able to keep the translation as concise as possible and keep the verb tense distinction:
Foi assim: It was so.
Era assim: It used to be so.