Not to mention that each piece of hardware is built with the assumption of there being extant suppliers for its component parts. For Apollo hardware, this is rarely true, so you'd have to retool and test for each part. The sad thing is it'd actually be cheaper to build a brand new Saturn-V equivalent than to make an exact duplicate.
This is actually one of the sorts of cases where 3d printing (no, generally not things like plastic filament extruders... meaningful printing, like laser sintering, laser spraying, etc, as well as CNC milling, hybrid manufacture techniques and lost wax casting on a 3d-printed moulds) has the potential to really come into its own: all of these sort of parts that you only ever need half a dozen of them made but might some day suddenly want some more a couple decades down the road. Another interesting advantage on this front is also that of incremental testing - I know of one small rocketry startup that has set themselves up to sinter out aerospikes in an evolutionary fashion - they print one out, connect it straight to test, measure its performance, scrap it and feed that performance data back into the generation of the next printout, in a constant model-refining process. Combustion simulations can be tricky to get right, but real-world testing data doesn't lie