I'll repeat once more the things you need to write a copier (actually
I notice I forgot to give one of them):

        a notion of equality of objects

        a method of saying if you've seen an object equal in the above
        sense to the one you are looking at now

        a method of knowing which slots of an object you need to
        descend into to copy it.  This depends on the semantics of the
        slots as well as their content (this is the one I failed to
        mention).

If your notion of equality is EQL or something similar, then the first
two things are done very well by a hashtable, if it is more general
then you need to write some hashtable-type thing.

`Knowing which slots' is application-dependent, but you can do stuff
with the MOP to find them.  I'd tend to do it with something like a
map-interesting-slots generic function with progn method combination
which I'd use to define the new interesting slots when I defined a new
class.