An engine swap would be the first step towards fail. The extra weight in the front from a bigger engine would not only compromise handling, but make spin-out on deceleration more likely.
The '03-04 Cobras weighed around 3650lbs and were pushing about 390hp at the crank. that equates to about 106.5hp/ton.
To achieve that ratio in a 2650 lb focus, you would need about 290hp, which isn't that far-fetched a goal for a well-built Zetec and would actually give you a tiny bit of pull on the Cobra until you're on longer straights (where the Cobra will still have the higher top speed).
Granted, getting that much power out of the motor and onto the pavement will cost you a solid chunk of change, but that's pretty much what you're looking at.
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