The problem with doing another transmission is that the DeLorean is rear engine and rear wheel drive, and the engine behind the transmission setup is basically the opposite of nearly every other car on the road. If you could fit another engine/transmission pair, it would be mounted backwards relative to the donor car, and you'd have five gears in reverse.
I'd love to get just about any other automatic in my DeLorean, but it seems there really aren't any practical options.
All of this means that you're probably stuck with the original transmission and can only really swap the engine. Others here have mentioned that the design of the engine cradle mean that V-style engines fit most easily. I'm definitely not an expert here (I went with the "simple" 3.0L swap), and maybe you could figure out how to fit an inline four. But you'd still probably have to hook up it up to the stock transmission.
-- Joe