Fiat have always locked together the remote key chip, instruments block and the ECUs. You might get away with "just" the 500 engine ECU but expecting that to interface with the Panda body computer is a big wish.
I think you need to factor in the full set of 500 electrical kit with all computers and instruments AND quite probably the wiring loom. Transplant the whole lot into the Panda and it should work fine.
You would be wise to take the 500 back axle and narrow it to fit the Panda shell (its 25mm wider at each side). ALternatively widen the 100HP arches. Why? Because it handles a whole lot better in ordinary 500 form so will be better still in the Abarth versions. If you are doing all that, dont forget the front end, suspension and brakes.
Ultimately you'll end up with a 100HP body shell with the Abarth 500 subframes, mechanicals and electrics. It would be very different and probably very good indeed. Only you can tell if its worth all the work for a modified car that won't have any resale value?