In Britain the lunatic fringe are putting big motorbike engines (R1, Hayabusa) in the backs of classic Minis - sometimes in carbon-fibre monocoque (unibody?) replica bodyshells. The benefit of the Mini is that it's so much lower than a Smart so it goes faster and handles better - plus it can have a passenger seat.
If the bodyshell is really light, the bike engines are so powerful that I bet they have a better power-to-weight ratio than that big V8 Mini - heck, they probably have a better power-to-weight ratio than just the V8 without the Mini!
Andrew