Unless you're gonna shoehorn a 3.2 V6 into it, all standard Busso put out way less power & torque than the standard motor in the 4C.
I agree with Bonno, much better idea to pony up the 1.75 already in the car, it'll weigh no more, and it wont screw up the handling.
I also doubt if it would actually physically fit in the 4C either.