Yes, you're right. Most of these games belong to the flat 32bit era. X-Com does not, so I already guessed that it was a completely new engine. Anyway, these are nice DOS jewels that can be played multi-platform today. X-Com makes me think that this is the best way to go: a new engine. My view is still that this would be the most stable and workable solution. But well, a some good things can be done on this source nonetheless.