I hate to start a bickering feud, but i would need to see time slips to believe you got into the 12's with a fwd cavy. Nos is a cool charge, and would have NO effect on your heads warping. If anything it would push your head gasket out, or eat up some piston rings. It could be many things from not retarding your timing, pounding your bearing out of wack, allowing the piston to slap the head / valve. Who knows with those dime a dozen 4 bangers.
My roommate had a 98ish cavy. It was a beater, neglected, and driven hard but it never broke down or had any major work done. Starter / alt is all i believe.