My Honda Civic 2007 won't start after installing a new engine because the first engine block bust out because water mixed with the oil. So I got a new engine. After they installed the engine, at the dash board at first the oil and battery light, brain box light didn't come up. An electrician checked the fuses and they all came up but the car won't start. Instead it is flashing the green key light. I have been using this same key for it and the key still locks the car and unlock the car and still working properly. An electrician has tested the fuse and said the car is not reading the key and needs to be reprogrammed. I still have this feeling the engine was not wired well or maybe they didn't fix some wire properly.
More than likely the problem is your immobilizer is no longer reading the chip in your key. This seems like a usual occurrence with Hondas when the green key light is flashing in a no start scenario.
The immobilizer ring goes around where the key inserts into the steering column. It should look like this:
This is where it's at when installed:
It's actually the part with the Roman numerals on it (or at least they are: I, II, & III ... whether Roman or not ;-) ).
I believe the part number you are looking for is: 39730-SNA-A02. It is my understanding it can purchased from most major parts stores.
I found this video which explains how to remove/replace one out of an Odessey. The basic procedure should be about the same for your Civic.