The Panda30 engine runs in the opposite direction to the 500 (110) and 126 (126A/126A1) engines. I expect if somebody was prepared to spend a LOT of money and time it might be able to fit it into a 500, but this might also involve gearbox bell-housing changes.
This is not the first time that Fiat have made what look like very similar engines run in opposite directions---the fiat 600/600D and 850 engines look very similar and in fact will bolt on the the same gearbox, BUT, they run in opposite directions. It is however,easier to make a 850 engine run backwards (or, as 1 poor guy who had not been told of this situation, found that he had 4 reverse gears and only 1 forward gear) than to make the Panda30 engine run in the opposite direction or be matched to the 500 or 126 gearboxes