There are two different airbag modules fitted to the fortwo and roadster, one being the standard twin airbag the other being the quad (for vehicles with side airbags)
Airbag modules can usually be re-used after they have detonated the airbags, however they are very sensitive to being dropped.
Twin airbags (pictured on the right) has the part number Q000 1211 V008 0000 00. This is a standard module.
Quad airbags (pictured on the left) has the part number Q000 0055 V009 0000 00. This is only fitted if your car has side airbags.