That is indeed an Alfa twin spark engine and it looks a pretty good fit in the Beta. It looks like the Beta fuse box has been relocated from the bulkhead to make room. I think the Fiat 16V turbo engine should fit, the Alfa engine is tilted forward slightly less than the Fiat engine. The Fiat's turbocharger will be very close to the radiator fan, but that can be worked around. I think the Beta rear gearbox mount can be used if the diff rear casing from Beta gearbox is fitted to the Fiat gearbox. Then you have to fabricate two front mounts to suit the brackets on the Fiat engine and gearbox.
Cheers R