Well, now that you do have some cold air, the vents are controlled by vacuum, did the vents work correctly before the engine replacement? If so, I would first start looking in the engine compartment for correct vacuum connections and leaks especially around the firewall, and if yours is an automatic check the vacuum line going to the modulator, if no problem is found in the engine compartment, then check the vacuum connections behind the glove box which is fairly easy or last but not least check the potentiometer but the dash has to come apart for that.