I guess Unit 1 was repaired enough to be servicable
To which by then would be a miracle and utilizing the spare parts to repair it, F89 Unit 1 would be one of the few remaining units that remain as is and isn't a mixing build, plus it'd be 50-ish years old during the story of Dust. The last hypothetical MS that was also an older one was the ZZ (Gump) itself under Old Judau in Victory Gundam Gaiden. Given how the setting of Xbone Dust shows us that:
A. You either make shit with garbage laying around from the wars in the previous years, which is what we basically see.
B. Lucky enough to own a MS that is still in tact, or refitting some to fight in space (like the Zgok in the early chapters and that Byarlant).
C. Or just go full mad max and make your own original, whether its stupid or not.
D. Some chances of semi original units that are manufactured even if in the era where things went wrong years after the Zanscare war.
Safe to say its a game of chance on treating it like a scavenger hunt.