the only remaining question is, are the stub axle plates the same?
We know for certain that the beams are different from the Fiat master parts catalog (they have different part numbers and the catalog says the 100HP one is for the 1.4 only). What we don't know is exactly how they are different, and what the significance of that difference is to someone wanting to fit a standard axle to a 100HP.
Ideally we need someone who has both OEM axles stripped down to take some careful and accurate measurements.
The aftermarket situation is, frankly, a mess, with about as many different views about what fits which models are there are resellers. For example, Imaxles sell the same part as being suitable for 500's both pre and post 2010 model year changes; there's a substantial difference in the torsional rigidity of the two beams and, IIRC, neither the springs nor the dampers are interchangeable - their website makes no mention of this.
The aftermarket beams may fit dimensionally, but I'm wondering how much thought has been given to matching the torsional characteriatics of the OEM parts? There's a lot more to making a twist beam than just welding up some pieces of steel that fit into the space.
As Koalar says, the twist beam is a key component affecting the way the whole suspension system interacts; a lot of thought and testing no doubt went into the original design, and just bolting something on because it fits, or can be adapted to fit, could have a significant negative impact on the handling and stability of the car. The OEM part may not be the best engineered component Fiat have ever made, but it has at least gone through Euro NCAP testing and type approval; I've no idea how the aftermarket parts would perform in such tests.
My take on this is that prevention is better than cure; if your beam is currently in sound condition, do what you can to keep it that way.
