That's always a tough call to offer advice on without seeing the cars. I think the answer to the question ultimately is: will you miss the 91 LX? Will you regret losing it? Either car could offer you mechanical issues. Who knows. You might have no issues with the SN95, but then you might have tons. Same could be said for the 91.
So really it comes down to how do you feel about it? I think both cars are probably equivalent for a straight up trade - or at least close enough such that if you get what you want then it doesn't matter. The SN95 is a better platform for tracking, but both could be tracked. I think, in order for you feel good about it, it really comes down to would you personally be happy with the Cobra and tracking it? Or would you regret giving up the LX? If you would be happy with the Cobra and tracking it, then I would trade and not look back. If you have personal or sentimental attachment to the LX, then see what it would take to get the LX track worthy - it can be done. I assume neither car would be your DD? What would sour the trade is any sentimental regrets. You should forget mechanical regrets - either car could at any second give you trouble (and once you start tracking it, expect trouble). Is tracking a Cobra worth a 91 LX to you? If so, do it up.