I was pretty sure that there were two plastic/metal through fittings that attached to either side of the hull and that a piece of tubing connected the two with some sealant on the ends. Here's a shot from the inside of a 1997 Nacra 20:




there's more views here...but they don't show the whole thing (it wasn't what I was trying to see at the time).

http://www.flickr.com/photos/teamseacats/440875980/in/set-72057594056084945/

You have a couple of choices:

1) sleeve it. make a fiberglass sleeve (or find a thin aluminum one) just small enough to slide inside the tubing that is there and seal it cleanly on either side of the hull

2) remove the plastic fittings (probably will need to destroy them, install a new fiberglass sleeve and fair it to the hull (making it look special).

3) buy some new deck bushes and replace what's there. Make some tubing to go between them (but be sure to run a piece of string from side thru tubing to the other side so you can get everything gooped up with 5200 and aligned without dropping stuff into the hull. run some expendable line back and forth through it to clean up the sealant and clean up the exterior with mineral spirits.

Deck bush:
http://www.apsltd.com/c-180-pushfit-bushings-by-rwo.aspx