As said , the aerial is a 15 min job at best. most ones of internet or maplin will fit. or some have used an sticky uppy amplified bee sting aerial to good effect also.
Any stereo will do as the car has standard din plugs fitted for the stereo. two things to bear in mind. one is that the stereo wont turn off without removing the face panel unless you modify the wiring to have a switched live from the ignition circuit. The alfa strangly has two perminant lives, why who knows, every other car has a switched an a perminant live. Possibly this has already been done by the previous owner if you can turn off the current stereo without any leds staying on. If not just keep on taking off the face off or do a 5 min mod in the wiring , do a search on the forum. A popular switch live is the cigar lighter on a p2 model, no idea where on a p1.
The other thing is you may have a gap around your new stereo. you an buy a bezel off ebay for a few quid other wise it looks awful, or again perhaps you can use the one off your current head unit.