On my one, I carefully took out the headlining making sure it does not get too damaged. Bought felt (about 2 meters). Made a template (traced the shape of the headlining onto the felting). I bought one of those orange plastic sheets that is used to tile floors (the name escapes me). This material is brilliant because it is so flexible. Cut out the shape of the headlining from the sheet (slightly larger is okay - i'll explain why). Cover/glue it (adhesive spray) with the felt, folding the excess (not to much excess) underneath. Install it by pushing it from the rear hatch into the same position. Slide both parallel sides into the crevice on either side. Slide the front into the front crevice above the sunvisors. But before you do so spray adhesive glue onto the roof surface. You have to do it right the first time, so practice sort putting it into place, until you are confident enough to put the thing into place. Reinstall the rear plastic screw plugs by making holes on the felt covered sheet. Make sure again the you spray the section under the screws for good measure.
Sorry, I couldn't find my detailed thread on this. It was quite a few years ago. It is still in place!!!!
Alternatively, there is a new one on ebay.
https://www.ebay.co.uk/itm/CIELO-HALL-ROOF-FIAT-126-BIS-FSM-ORIGINAL/112833411885?hash=item1a45653b2d:g:yGgAAOSwRCpakCwMGood luck!
