You're wrong about the year catalysts were required... Its 1992, not 95.
If its pre-92 you're fine without one, though if emissions are a concern and its possible, fitting one for the test will make a very big difference. Someone on here will probably be happy to help a fellow alfa owner and give you a lend of theirs for the day
Fitting a temporary silencer is fine, as the mot tester isn't allowed to dismantle any part of the car to examine it. So as far as he is concerned your temporary silencer is permanent.
There is an option for them to fail a car if 'the exhaust is significantly louder than a car with a standard exhaust' (or something along those lines) but only the worst of testers will use it, unless you ruffle their feathers somehow
I prepare cars for mot and put them through the test as a sideline for pocket money and from what you've said, your biggest concern is likely to be the emissions. As an older car, the limits are a LOT more generous (up to 0.35 iirc) but its not uncommon for an old tired engine to go way over that.
A good service and a warm engine will do wonders though, take the car the long way round on the way to the test and make sure its up to temperature... If possible, ask him to do the emissions first, or if he wont see if he will leave the engine running while he does the rest of the checks.
And most importantly, be nice to him
A bit of common decency goes a long way. Make use of the viewing area (if they say they dont have one, walk away, its the law) but dont act like a scrutineer as it will annoy him... Just show an interest in the car and his job, be an enthusiast... Do all that and you never know, he might overlook that warning light on the dash, or the brake light that's out. If he does, chuck him the price of a pint AFTER the test as a tip, and go back to him every year... He will remember you