I had the same problem, and after several inquiries the easiest solution was to buy another Italieri kit, remove the centerline pylon from the tank, then fill and sand the cut surface. Attach onto wing pylons based on photos found on the net.
As for aftermarket, there is actually noone who makes these 1200L Mirage F1 tanks - strange actually, as the Mirage F1 look drop-dead gourgous with them
The other F1 was buildt up and installed with an FM centerline tank - the banana-type of 4000(?) litres.
Both still pending decals, as too much work the first half of this year left me without spar