From what I have gathered here, which with $2 will get you a bad cup of coffee, 483 was set aside simply because it needed more work than bringing the unused engines back into service. Ever since, whenever another engine has been needed, the same has been true.
It's not that 483 is so bad, or even that so many parts have been "borrowed" from her, just that she is always the highest cost option, other than 494 or 495.
Assuming nobody donates a cubic wad of cash earmarked for 483, my understanding is that, should another engine be needed any time soon, 492 and/or 497 would most likely be the cheapest to restore to service. (or some combination thereof, but that is a different can o' worms.)
So 483 sits.
hank