All those modules do is govern how to build a Terror Star- build the four in a chain and voila, Terror Star, baby! I believe you could even make it more difficult to build a Terror Star, as the % for the 4 modules add up to 100%, signalling completion. I imagine you could stretch that out, I haven't tested this yet though. Or even speed it up.
But they don't change any of the attributes of the Terror Star. Nor is there an entry for the Terror Star in the ship files.
One might want to change the Terror Star graphic, but as BoogieBac says that's possible with the encoder.
It starts life as a SB, so you could add new modules to beef up the defense/culture/productivity, so no real need for new stuff there.
What's left? Speed, maybe. That's hard coded to 1, for balance reasons. Takes forever for those things to get into position. Or maybe 2 different flavors of Terror Star?
~SDC~