if it broke left half, right half you might get away using epoxy to put it back together
but if you mean top half, bottom half you'll need to
1) buy new - very expensive i guess
2) buy used - might be very hard to find, but i seem to remember a dutch cat breaker who may have more nacra stuff that most
3) get 1 made by someone who knows their stuff, carbon over foam core?
4) knock something up yourself from carefully planed, sanded plywood and then wrap it in several layers of glass and epoxy, then filler and sand it smooth for gelcoat or painthttp://www.catamaranparts.nl/http://2ehands.nacrasailing.com/dealers.php#37