the standard alarm initially was a Philips aftermarket alarm, put on by the importer in Dover (along with 75 skirts, wheels etc) to satisfy UK insurers. Later a proper alarm was fitted in the factory, which is what the set of leads are on yout spare wheel.
Now it is possible, because I have done it on my car to fit an alarm off a facefift,a receiver off a boxer 145/146 (the thing up by the interior mirror) and a remote fob off a 155, link all the wires together, they will all be there and get the system going. Why the different cars? well the later cars have the remote built into the keys, but you cannot transfer the chip out of your old key for the CODE system to start the car or the metal key for the ignition barrel and put them into the later keys, hence the recipe of Alfa parts.
Trouble is the system is based on the early infra red technology rather than later radio waves and it is a bit weak.
However, it will work and the parts are cheap enough if you want to do it. The bigger concern is taking out all the stuff previously fitted.