Not to mention there being a substantial advancements in engineering that make the F1 engines used by Saturn V basically obsolete.

The Space X Merlins have a much better thrust-to-weight ratio in the same gas generator cycle class, the Russian RD-180 powering the Atlas V is using oxygen rich staged combustion and is much more efficient.

And the current trend seems to be clearly liquid methane and liquid oxygen, covered by the phenomenal Raptor engine from Space X, the BE-4 from Blue origin and many smaller ones.

So hardly any regression on the chemical rocket engine front - pretty much the opposite, thankfully!

On the nuclear thermal rocket front on the other hand - yeah, we really did regress there. :P From almost flight ready NERVA examples in the 60s/70s to basically nothing even remotely flight ready today...

IIRC we have (at least most of) the drawings, but they don't specify tolerances like modern drawings do and were more for reference as parts were developed at specific factories with existing molds. It's more the fabrication knowledge that would need to be rebuilt.

In addition to that, a lot of stuff was crafted by hand with little or no documentation as to what was changed (no 'as-builts').

e: How NASA brought the monstrous F-1 “moon rocket” engine back to life (https://arstechnica.com/science/2013/04/how-nasa-brought-the...)