First thing I'd do would be to get 3 AE86 Truenos.
In one of them, I'd stick the F20C (S2000 engine), complete with ITB's and titanium exhaust

- That would sound INSANE!
On the second one I'd place a rotary engine (preferably a 4-rotor)
Third one - 20v 4A-GE (also complete with ITB's and titanium exhaust) - 11.000 RPM's with the ITB's induction sound is just car porn!!
Also, I'd like to place the Ferrari 599 engine in a smaller coupé... E92 M3, or even better, the 135i.
I'd also place the most inefficient engine in the game, with the highest fuel consumption in a Prius, just to completely ruin the whole point of that horrid, horrid car
EDIT: I'd also like to place some decent engine in the DeLorean.