After doing some research, here's my opinion on this:
The original belt on a 103 with air pump should be 2415mm. Without air pump it should be 2257mm. The correct part number at fastlane is either the W1000-21017 (Conti) or the W1000-99033 (the other one). No idea why you are coming up with 2335mm. Maybe it's due to the problem you are having with the tightener.
On the research I did on the air pump question in general, it appears 1990 was the first year, as there is no reference to the air pump wiring existing until 1990. I think the problem you are having with fastlane is because the 89 is lumped into the same year range as a 1990, which does have the pump.
I am really pretty sure the numbers I gave will work. It seems to me when I was contemplating doing this on a car that it looked like the air pump can stay bolted up too, if you don't want to remove it. The belt will miss the air pump pulley entirely. That's how I recall it anyways.