MCC Smart Car manages well on a 3 cylinder 600cc or thereabouts turbocharged engine.
Just dismiss the concept of 'fitting' or 'adding' a turbo. Yes, turbocharged engines are derived from naturally asiprated engines (though some makers, MCC Smart, Saab and Volvo) offer a whole range of turbo engines with no naturally asiprated models), you have to consider that a turbocharged engine has many many differences, internally and externally, over its naturally aspirated counterpart than the simple addition of a turbocharger and (hopefully) an intercooler.