What about the engine from Just Cause 2, it had room for large aircraft and every other aspect of the game. But I do see a problem with the flight engine in the game. Zeppelins don't require power to fly, which means if the engines are off (or destroyed), the zeppelin can still stay airborne for some time. The engines are simply required to move the zeppelin. There was a WWI aircraft package that came out for Flight Simulator X that included a WWI era zeppelin, but it used the flight characteristics of a slow flying aircraft. It seemed like it was realistic in motion, but flew like a large aircraft, which isn't very accurate zeppelin-wise. Yet, this is only a suggestion, and after checking your post on the GTA forums and still seeing no response for almost a year, I'm pretty sure you should find a new direction now.
Montana Zeppelin & Radio Company