Since the ATV (Agena Target Vehicle) used the Atlas as the launch vehicle, most of the work was already done. So after researching the ATV it was time to build. In fact the ATV itself only took one try (besides a few minor changes to fuel balance and thrust limits)
After the ATV was created all there was left was to integrate it onto the Atlas vehicle, which was quite simple
Now all that was left was to tweak the fairing (the nose cone that protects the ATV from the rigors of ascent) and Agena was ready to go
Finally, Here is a comparison of my Agena to the real one (as well as the Agena-Atlas)