On the cinqs, the FIRE engines are 1108cc 54hp, there is also a 1242cc version in the punto that is the same overall size but slightly taller, available as a 60hp single point injection, 75hp multipoint or 90 hp 16 valve. Newest 1400cc eg panda 100hp also have similar overall dimensions. I have a standard cinq sporting and I think its a great and durable engine, although I generally intend to go 1242+ turbo when I get around to it.
If there isn't room to keep that radiator, think about a front radiator conversion. I haven't noticed this being a common theme with the 126 bis guys but I was into Imps years ago and it was the only sensible solution to the cooling. Just means putting a radiator where the spare wheel was and venting the front panel and either the spare wheel well or the bonnet to let the air in then out. Some plumbing involved and you end up with quite a bit of heat inside the car as you have a pair of large hot water pipes running through the cabin.
Another point on this thread is the question of engine rotation. I don't know if this will work with the fiat transaxle, but another thing the imp people did was turn the transaxle upside down to reverse the direction. In their case it was because some people turned the whole engine/transaxle installation around to create a mid engined car but you then have to flip the transaxle upside down to correct the direction (4 forward and 1 reverse gear). Needs some consideration about the oil filling, draining and venting of course.
That's about all I have to say about that.......