Hi
sorry to be a bit late into this thread..
A few questions about your Panda/Marbella
Which engine is it? (Does the exhaust pipe go down the front, or the back of the engine) Pipe down the front is the "FIRE" engine - used in Pandas, some Unos, some Puntos... Pipe down the back is the 903 engine which is used in Marbellas, Pandas, some Cinqs, and some older FIATs (it's an old design).
Depending on the engine is what sort of filter/induction choices.
BTW your BHP is likely to be 45, or less, as manufactured - but don't despair: when running at their best they can still surprise people.
To give some idea, my eldest daughters CL ( 45 "FIRE" ) does 40 to 60 in 4th gear in a bit under 12 seconds
As far as I know it is unmodified.
Have a look at
http://www.carsfromitaly.com/
and navigate through to the FIAT section, then MODELS.
Regards
John H