If anyone has seen the "random ufo design" from XcomUtil, then you know it was a great idea badly imiplemented.
What I suggest is simple. Keep ALL ufo designs, maintain the originality of the game, but randomize the "direction" it faces.
So you have 4 versions of the same ufo, facing N,S,E,W. And if we can't draw the "back-side" of the ufo entrances then just facing South and East is already doubling the number of ufos we encounter.