Afaik:
# IC designs are covered by copyright law. But that covers an IC's specific design (transistors as layed out on the die, artwork etc). So verbatim 1:1 copying is a no-no.
# Their function may be re-implemented if you design that re-implementation yourself. But some aspects of an IC's function may be covered by patents. This is why eg. there are no 3rd party x86 vendors apart from AMD/Intel or their licensees: leaving out patented bits would produce a non-competitive chip.
Read: with IC for which patents have expired, reverse engineer to determine function -> re-implement using newer tech, is most likely ok. For a recent-ish chip, you may want to skip functions still covered by patents.
The line between "reverse engineer to determine function" and "copy original design verbatim" is fuzzy though. That's where a cleanroom approach comes in (as noted by ce4).
In case of the 6502, I wouldn't sweat it. :D
