I have fitted a Walbro pump which is normally found in a Ford Escort RS2000/Sierra 2.8V6 XR4X4 etc in a Spi turbo Cinq.
I have also experimented with the built in regualtor and it can run upto about 1.8bar before locking out, therefore no matter what pump you fit it will never run more than this, it may flow more litres per hour, but never higher pressure. The Fiat manual states that the regulator is set anywhere between 0.8-1.2bar as standard, I would get a fuel pressure gauge and measure what your car actually runs and then adjust up to say 1.5bar and see the effect on the car as you may find (what i suspect) that the standard pump can run much more than 1bar, as its a Walbro pump found on Spi Cento's.
You could remove the regulator and run an external one and run more pressure with a bigger pump.
I ran my injector at 1.8bar no problems, but backed it down to 1.4 as i didn't nee dthat much on the turbo as i was runing 2nd injector at 3bar.
The MPi pump will fit as well, there are any number of pumps that will fit TBH if you do your research.
Aaron.