06 audi a4

EXACTLY....

It has to be the same engine. If not the computer would get the wrong signels,if it got any at all. In the 1960's,if you knew how.you could have changed to anything,but those days are long gone.

I'm pretty sure if you have the cash and the know how you can still fit any engine in any car. Now a v12 in a Smart for 2 (Smart-IV-12?) might be excessive but still doable. I'm not sure where you would sit unless it hangs out the back or powers a trailer from behind. I say go for it. Of course it probably won't pass your states emissions or safety checks. But you asked if you can. Yes. Btw in Europe Audi offers many different engine options compared to the US.