I would think it's alot easier to replace component modules and hull plating
than it would be to build a ship from scratch, assembling the entire ship
infastucture and framework.
But we are talking about actually replacing areas that aren't there anymore which means, reconstructing infrastructure and framework on the frontal saucer section.
You saw the extent of the damage ...
And most of the damage was already repaired. Numerous hull plates were missing indeed, but it wasn't incredibly much.
As for the time frame being a month.
Color me crazy, but in my opinion that's way too much time for an officer to be debriefed and to depart to his next command.
By that analogy, it must have been decades for Janeway to undergo debriefing for her 7 years of being in the DQ.
Scanning B4 wouldn't take long at all ... give it half a day for thorough examination.
Besides, Picard wasn't entirely sure what to do with him either and left things open in that regard.
Deks,
When I talk about scanning B4, I'm not talking about running a tricorder over him for a few hours... I'm talking really in-depth stuff, like bringing Cmdr. Maddox on board, and having him and Geordi do a comprehensive breakdown, testing, and workup of all of B4's structure and ability, as well as testing, to see if somehow, B4 can be "expanded" to something resembling Data's level of AI. Basically doing an in-depth study of B4.
Also, you have to take into consideration the time frame... we can assume that this movie takes place very shortly after the end of the Dominion War... you of all people have to know that Starfleet's shipyards are gonna be inundated with work overflow for a LONG time, considering the epic amounts of ships that were lost in the war, and the massive rebuilding that has to be taking place. Yard crews will prioritize, yes, but it's also a matter of equipment availability, yard team availability, parts fabrication speed, etcetera. I really don't think a month is out of the question at all.