Big power from little cars.
Unveiled in 1972, the Fiat 126 served as the replacement for the original 500, and as such, it is a very tiny car. It had an appropriately tiny engine as well, but the following video shows a couple of 126s which have had their engines replaced. The red car is from Taz Racing, and has a Chevy V8 stuffed in there somewhere. We're not sure exactly what's powering the blue car, just that it's a "Toyota twin-cam" engine. The exact specifics aren't known, and neither do we know how they got those engines to fit.
But it's safe to assume that they're now a bit quicker than stock.