The Bis in actual fact had a limited run. They sold watercooled versions from 1998 to 1992 in the UK and elsewhere. Personally, watercooled versions are fine. There are quite a few advantages of buying a watercooled Bis. The engine is quieter, more practical (hatchback plus additional luggage space), electric wash/wipe, better alternator, bayonet fuses, superior heating system, headrest, larger wheels (so has a smoother ride), gear ratios, just to name a few. However, you need to keep an eye on the cooling system and water gauge and ensure that the automatic electric fan works.
There are from time to time a number of UK aircooled (AC) 126s being sold on ebay, Car and Classic, Gumtree et al. Just keep on looking at these websites. BTW, one of the biggest advantage with AC, is there are lots of hop-up/Abarth parts for it.