You could look at it this way, whatever you replace that main woofer with, it will most likely sound better than the existing woofer
Honestly though if you are after anything that represents good clean quality sound the odds are against you to find a good match that is different than OEM replacement parts.
I'd scrap the boxes and start from scratch. I tried to find a cost effective driver to match a pair of damaged polk audio RT3's and ended up scrapping the whole replacement idea and building boxes from scratch. The new speakers sound very good.
I'm not saying it can't be done, just saying its tougher than you think. It was for me atleast.