Ah, something I know a bit about!
Sort of…
Depending on what your budget is, for powering a car you’d want something with a bit of bottom end on it, I think that’s why the ‘Busa engines have been popular for the Mini’s and Seven’s. Having said that, the GSX-R 1100 oil cooled engine is still very well regarded and has a stupid amount of tuning options available for it, I think most of the drag bikes are still using them. Though it would limit you to its 5 speed gear box (could be wrong on that though, working from memory here).
Another cheaper option maybe a ZZR 1100 engine and ‘box, I think they bore out to around 1400cc, but you’d want to check on that.
Other things I can think of, making sure the Fiat’s shell is strong enough to have the mods done to it, and be something you’d not kill yourself accelerating in. But that part I’m sure you know already.
Right, back to lurking for me.
Cheers,
Mat