None of those things present any type of tuning difficulty. The power and torque curve of the engine is well documented, check. The suspension geometry hasn't change, adjust spring and dampener rates, check. Adjust final drive, gear, and diff ratios, check.
Hell, other then setting a custom power/torque curve, we as the end user have the ability to make all those other changes in game. You really made the differences out to be way more than it is. The cosmetic changes would be more work.
I mean, it really does...
The suspension geometry *is* different. The Fiat runs lower as a singular example, not withstanding the damping rates etc. will almost certainly be specific to the Fiat. The power and torque curve is documented, yes, but that still needs accurately modelling as the engine isn't in the game in any other vehicle AFAIK - and you have the sound modelling that would also be required. Then you need to take in to account whether they do the standard Fiat 124 or whether they do the Abarth one, which is completely different again.
I agree the cosmetic stuff would be more work, but what you're failing to realise is that they need to make an entirely new vehicle. This isn't some GT86/BRZ situation, what
you have described is
every single parameter that could need programming to make a brand new car (full outside and internal poly model, new suspension model, new engine model, new gearbox model and new sound model). The level of work required to create the Fiat is likely MORE than creating the BMW Z4 M40i for example because the Z4 engine is already present in a different vehicle in the game. #BMWSupraYo.
So, no. It isn't simple, quick or easy and is likely MORE work to put the 124 in to the game than some of the other cars on that list.
EDIT: Have looked in to the finer details, the gearbox is the same unit between both but the rear differentials are different. Suspension is different, engine is different, styling is different, exhaust system is different, one is turbocharged so requires drastically different audio recording, they come with different tyres from the factory... I can go on?
Also the Fiat lacks an LSD, which requires different modelling also.