Getting high on hybrids:
As your airships get faster, they will need bigger and bigger engines. But the shapes will also change. They will need to increasingly look like lifting bodies, with aerodynamic shapes moving to the forefront. They'll probably need to adapt to high atmospheric conditions, since the thinner air will allow less drag. In short, they'll need to look and behave more and more like conventional aircraft. It might even be difficult for these airships to lift off on their own buoyancy (flirting with the edge of being more hybrid airships, which they would probably work well as).
Since they do not need to lift their own weight, they may increasingly look like bullets - elongated to reduce drag and friction. Jets would likely be the best engines for these fast beasts, and rockets could work well also.
But as you get faster and faster, you need to ask yourself, WHY? Your airship is going as fast as a conventional aircraft, looks more like a conventional aircraft, may need to mass near that of a conventional aircraft, so why not empty the bag and just make it a conventional aircraft? Perhaps you could have an inflatable ammonia balloon system to lift off and gain altitude, or help with low-speed operations. But once you want to go fast, start speeding up, condense that ammonia into storage, and just fly.