Has anyone put the differential from a controlled slip Viscodrive-type system from the Fiat Coupe 2.0 20v Turbo with the transmission system of a six-speed gearbox from the Fiat Punto 6-speed or even the 5-speed?
What mods do we have to do to attach both systems?

No but toy may want to be aware of a problem/feature you may not like.

Due to the Viscodrive system fitted to the Coupe 20V Turbo it had a turning circle of over 1 metre MORE than the Coupe 20V (naturally aspirated).

I've never research mechanically why this was the case and maybe it requires a steering rack change.

What I do know (having owned a Coupe 20V non turbo) the turning circle was not great for a car with such a low front and long over-hange and many a 20V Turbo owner have been even more disappointed with the 1m+ added.