Just replacing the drivers wont really work so well. The crossover is designed specifically for the drivers, the enclosure and multiple other factors. If you want to build your own there are tons of excellent DIY designs. Many of these designs will deliver a great sounding speaker for a modest price.
Check these out for starters.
http://www.htguide.com/forum/forumdisplay.php4?f=39 http://www.partsexpress.com/projects.../homeaudio.cfm