im not really sure what kind of tuning mods you can do to the 903 or 899. the older 903 with it being carb i guess could have bigger carbs and the like stuck on it.
the 903 and 899 are basically the same engine they fit both the same mounts and the gearbox will fit both 903 and 899. bottom end wise, i could not spot one difference between the 2 engines.
to get the old 903 running on EFI you'd need to find a way of attaching a crank position sensor to it. you can make a bracket out of some stainless (because its non magnetic) and have it held in place by a few sump pan bolts. then you just use the auxiliary drive belt pulley from off a fiat cinquecento 899 engine as the sensor uses that to detect when the engine is at TDC.
thats about the hardest part... just make sure you get an ECU from an early cinquecento as they dont have an immobiliser on them.
you'd need the water thermostat housing from off an 899 engine as it contains the water temperature sensor that the ECU uses.
you'll need to use the rocker cover from off a 899 cinquecento as that had part of the adaptor plate built into it, to make it so the SPI throttle body fits the engine. you can however, just cut this off the 899 rocker cover and have it stand alone and keep your nicer looking 903cc rocker cover. (this is what i was going to do)
oh and you would need an in-line fuel pump at 1 bar of pressure.
after that.. its all easy. simple plugging in connectors.
and to wire the whole thing in is only 4 wires!
large red wire goes to the battery, small orange wire goes to an ignition live (or old coil wire) the white with red stripe goes to the "injector" light (you can leave this disconnected its like the "check engine" light) the white with brown stripe wire goes to the fuel pump (i have seen this other colours though, sometimes green and blue or green and red its quite a thick wire with a single connector)
see how simple it is!