I had a late 1980s Nacra 5.7. It used a Harken small boat traveler car. I bet yours is a Harken, too; your old car might even have the model number on it somewhere. When mine blew up 15 years ago, Harken sent me a new one for free, but I imagine times have changed since then.

P.S. - If your traveler car is intact but not attached to the track anymore, you may just need to buy new bearings for it.