You know, though, the one thing about the colossus that is only possible with the colossus is to construct things within its "massive" fighter bay, which is probably the size of a couple aircraft carriers itself. So technically, you wouldn't have to make the reactor outside the ship first, just get it to where it needs to go through the ship, and the outside. And it's only one ship to upgrade, so only this ship would have to be redone.
I'm not saying that it's a good idea, it's just not quite as difficult. But it would probably be really inefficient to try anyway.