I don't know from where the replacement gas tanks your dealer mentioned came from.
Mine fit very well, as it is a fiberglass unit the rubber spacers holding the rear side clearance of the tank has to be reduced some mm on each side and the in the rubber spacers holding the gas tank in the front, (where the bolt goes through), has to be broaden some mm on each side. there is no shortage to the full steering lock! So I assume the fiberglass tank to be exactly identical to the original, the walls of the tank might be a little bit thicker on the inner
side as original, but else fit extremly well. I can post some pictures if you want, (warning graphic content, I already said that I' am stuck to Bul's).
I've got the gas tank from Spain, another ressource as I know is UK, the plastic gas tanks, (probably the same manufactor), here in Europe as I have seen and heard normally fit spot on. The fiberglas unit has the advantage that you can paint him but the drawback to not be so resistant against scratches and more sensible to impacts.
Very important to use/get the tank with the right logos and emblems:
To the plastic tank the logos should be printed on, to painted gas tanks Bultaco used transfer pictures, (shown below),
not decals (shown above).