You can have as many US parts as you like, so long as you don't have over 10 foreign ones (from the list [you know where to find it]).
You'll be fine on the WASR, Century leaves the fine Romanian wood in place for all to admire.
Swapping the "crappy poly furniture" on the Century AK74 may require some more questions. Is it Tapco furniture (first thing that came to mind)? Original Bulgarian poly military furniture (if it's this, it's not crappy - maybe just not to your tastes)?