Hate to throw cold water on the idea of pursuing them, but in addition to the things that the other folks mentioned like difficulty of proof, variables in installation etc,. Even if you were somehow to succeed, their is probably fine print in their warranty that states that even if they are liable, their damages are limited to the cost of the parts sold. They almost assuredly limit their liability for labor, incidental and consequential damages.
By the way, as a side note, the last couple gasket kits I bought from felpro contained only the rope seal. I had to buy the rubber from a separate supplier. So apparently, felpro is being proactive to avoid further future claims by getting out of supplying the seals.
If I were you, I would forget about the suing idea, and would instead call fel pro and see if they would work something out with you. You never know sometimes a company will help you out. Especially if they recognize they had a problem (why else would they quit selling the seals). Good Luck. Doug