You cannot remove the Cat because, as you've already found out, the computer cannot cope with the resulting exhaust mixture. You'd have to somehow get a hold of a reprogrammed computer which would probably cost more than a Cat (especially if you can get your hands on a used one). Removing the Cat on a stock engine does nothing to enhance exhuast flow either (although yours was probably melted and plugged up completely). The Cat has many more times the volume than it needs. Bet you also need new O2 sensors or at least the front one.