Ace Custom: Making these is an important part of the games.
The second game had its source code released some time ago, and a relatively new overhaul called Omnitech is being worked on. The Mechs that can be purchased will also change depending on which faction you're currently working for.
The main campaign is divided into three parts, and the player alternately works for a different faction in each part: Steiner, Liao, and a group of rebels supported by Davion.