Transfering computers with engines?

Okay so basically I bought this car from a guy. Its a civic, and the engine light comes on with codes that do not make since. For example, it says that the "Catalyst System Efficiency Below Threshold", and it has a BRAND NEW cat in it. I think the source of the problem would be the computer. The car is a 2002 honda civic LX. They took a VTEC engine out of a 2005 civic (I'm assuming si), and put it in the car. I'm not sure if they transferred the computer with the 2005 engine. If they did not, that may be causing the problem. How do I tell if the computer is from the original engine, or if they did transfer it from the 05 engine? If they did not transfer it, how much would it cost to get a computer?