I did something similar,from 1.4l to 1.8l.(Plastic top)
The engine fits right in except for a small difference in a coolant pipe.(fixed with a piece of pipe)
The gearbox of the 156 wont fit but you can use the gearbox from a 155 or another 146.
You will also need to change the 2 to 1 piece of the exhaust,gear lever with tunnel,ecu,imobiliser,ignition lock and key,maf,2 pieces of air induction pipe and aircondition hoses.
The 2.0 also has larger front brakes but i didnt change mine so i dont know what you need.
The rest of the exhaust,air induction,radiator,fuel pump etc are the same for all models.
Hope that helps.
P.S. If i could do it all over again i would go for the fiat coupe turbo solution