I'm not sure this is the exact spec, given install on the J18 in mid 90's, but the Ronstan Series 19 C Track should work. You may need a new car too, though. Just measure the dimensions to make sure you get enough length.
EG:
Alterrnate thought:
If i'm being truthful, though, I rarely move the jib cars anyway (usually at the end or begining of the season to make sure they're not corroding). Perhaps it's my jib cut, but I just have them set to full aft position and don't really think twice about moving them while racing/sailing... so instead of replacing the existing track, you may have enough salvagable track left to pull it off, cut off the bad seciton and flip it around to reattach it. I'd simply use the rear positioning and seal up the other holes from the track.
Good luck!
-Brian