1997 Honda Civic Gas mileage

Hi,

We have a '97 Civic with 154K miles that recently went through a number of upgrades. We swapped in a low-mileage engine from a salvage yard (32k miles). We did a pretty thorough tune-up at the same time (new oil seals, new plugs, wires, new O2 sensors, new air filter, new fuel filter, new belts, new H2O pump, new engine mounts). We also recently purchased a new set of tires. This last fill-up, I noticed a significant decrease in fuel efficiency, which dropped to 24 mpg from about 28-29 mpg. My car also recently recorded an engine trouble code P0420 which appreas to be related to the efficiency of the catalytic converter. Any idea what would cause the drop in fuel efficiency? Plugs and tires can affect fuel, but will it make that much of a difference in fuel economy if, say the spark plugs were incorrectly gapped? Is it possible one of the new O2 sensors is bad?