If it's a "spare" engine the parts are far more accessible than when the engine fitted into the car. The question surely is, "why, with the engine out, wouldn't you do it!"
Would anyone really fit the engine then live with the worrying niggle that the belt may let go and then change the belt after fitting it into the car. Surely not!
Well, I agree. I think I'd change only the belt and keep the bearings, provided they still feel like new.
But just for a different perspective - to change properly you need camlock tools (readily available in England; not available in some countries and a week to post them may be a problem), plus the cost of all the replacement parts (belt, tensioner, idler, balance belt, tensioner) at the equivalent of 350 pounds over here probably exceeds the value of the second hand engine
Otherwise I agree, good time to change it with the engine out. I did mine in my 156 using camlocks from England and it is nice to know it's done. For my Spider I bought last month, I have a receipt for $1950 (about 900 pounds) to say that it was replaced in February by the main dealer in Wellington, NZ. Worryingly, there are correction-fluid marks on the cover by the pulleys - makes me wonder whether they had the camlocks...! thus why I like to do jobs myself. EDIT: to be fair, they did replace the water pump too for that price: water pump price shown as $488 (240 pounds! I hope it has a gold-plated impeller...)
This is why Alfa Romeo has a reputation for needing expensive parts on this side of the world, and to have the replacement interval cut in half is rubbing salt into the wound for most owners, I'd guess? I think that halved interval is the real problem here...