Well back in the day when turbo spi were a bit more commonplace what was done was a diode is put into the map sensor wiring (its a map sensor not a maf, they are different things
) so that the factory map sensor doesnt provide the ecu with a signal telling it that there is positive boost pressure and therefore the ecu doesnt throw a wobbly about it.
The 2nd injector that is controlled via mf2 or whatever other device used has a signal from a completely different map sensor from a turbo car so that it can see the real manifold pressure and apply fuel appropriately.
This does work, I have owned a cinq turbo and sei turbo with this exact setup - both had mf2 but different versions. One had an onboard map sensor so it had a vacuum hose directly to it and the other had a seperate map sensor in engine bay feeding it a signal. But, this is not really a good way to control fuelling, its really a bodge - the mf2 was never designed for this purpose really, its primary function is to control water injecton.
Really the better way of doing this would be to use an mpi head and fuel rail (either Punto 75 or from a later seicento/punto (basically any fire engined fiat after 2000 x-reg), uprate the injectors and get either a good piggyback system that completely takes over the fuelling and ignition. I recommend a ECU Masters DET3 or a Greddy Emanage Ultimate for this setup, both are very good piggybacks and can be properly mapped to ensure safe operation. I would add that the Van Aaken conversions not only had the mf2 but also the ecu chip changed which retarded the timing a little - but as this is a well engineered bodge rather than the proper way to do it the retarding of the timing was not really referenced against boost as it should cause the ecu couldnt see the boost. Been a long time since i read through all the Van Aaken paperwork but i think it was done by engine speed and throttle position.
Or you could go full aftermarket ecu but thats alot more expensive.
Also take into account how much boost you want to run, as you might want to consider lowering the CR. On a 1108 easiest option is a decomp plate. If you have a 1242 8v you can drop the 1242 16v pistons in and get a CR of 7.8:1 - cheap but bit low CR really. For reference stock CR on the 1108 is 9.5:1 and the Van Aaken high boosts were lowered to 8.5:1 - the Van Aaken low boosts didnt change the CR at all. But from experience even if you plan low boost you'll want to crank it up, its inevitable so I always suggest lowering the CR in the first place.
I have to say at this point, if i were to build another boosted cento i'd just get a tjet these days, seems silly not to.