The Giulietta battery should have a rubber cover over the top of it, so it is possible to "hide" a more modern battery in the boot.
Alfa engines don't like little batteries, buy the biggest you can get to fit into the battery tray - make sure the terminals are on the correct side. Only buy a black casing battery. Warm the manufacurers label up with a hair drier & stick it on the reverse side of the battery, facing the inner wing.
Buy one of Bill Gillham's 101 decal kits and attach the Magneti Marelli decal to the visible side of the battery. (Bill also has 750 Normale & Veloce decal sets available)
There are other decal offerings out there, but Bill is the Giulietta Guru & I would trust his stuff over the others.
Bill Gillham =
hooligan@proaxis.com
Ciao
Greig