I suggest either a AWD Twin-charged Group B piccolina or a piccolina based on the many hayabusa powered hillclimb cars complete with insane aero and noise.
Im pretty sure there is no way you are gonna get AWD to fit there, or a twin turbo in an air cooled engine. Maybe engine swaps would be more likely tho.
There seems to be enough room to fit a drive shaft but the transfer case and front diff might be hard, also twin charged as in supercharged AND turbo charged like the Lancia Delta S4 although i agree keeping air cooled would not make much sense
I wouldn’t mind the SBR4 swap to be in the official game. There’s a few mods from team members I wish were in the official game, I really don’t understand why they don’t just put stuff like that in.
This may look like a game but it's actually a soft-body physics engine, which is a beefed up simulation game.
Rear luggage rack: Wicker suitcase to that of course too. http://mrfiat.com/italian/fiat/fiat-600/engine-compartment.html Also some Abarth bits that are not in vanilla content, there are quite few interesting pieces here: http://mrfiat.com/italian/fiat/fiat-600/abarth.html