The mk6 Zetec S shares the same 1.6 16v as found in the original mk5 Zetec S, but has more electrics built onto the engine and a different wiring loom not making it as easier as the mk5 to just drop a the Puma engine in for an extra 20 bhp. Although there are many modifications available to buy. One engine swap which is possible is using the 2.0 engine out of the Fiesta ST, but with the time and cost involved its normally easier to just buy a Fiesta ST in the first place, but it can be done.