Can I replace my Catalitic Converter with an after market one?

nope. will never meet oem specs. Roy

yes you can

Guess we need a tie breaker. In some states this is illegal, check with yours. No matter the legality, you will likely have nothing but problems with the aftermarket cat, they aren't built to the specifications of the OEM part and it will have an affect on the emissions system, which means the check engine light may come back on.