Three issues of concern here... the tampered emissions system, what part of the title transfer process is your Dodge Stratus in with the DMV, and has the current registration expired.
Once you return your Dodge's catalytic converter configuration to its original condition and re-position the oxygen sensor you'd have resolved the tamper problem (assuming this is all the buyer did to it). The question then becomes has he/she already started the registration process with the DMV. If the answer is yes, then the vehicle is no longer eligible for retirement. Similarly if due to wanting to sell this vehicle you let the registration lapse, once again it will not be eligible for retirement.
