OK, the best seats to fit would be Ford Escort RS Turbo seats for cheapness and availability, they are recaro items and so with a little angle grinding of the rivets on the seat frames, these will bolt straight to the floor, you will need to drill / grind the runners from the floor of the 126. I'm just in the process of doing this now so will let you know and let you know with photos how it went, I know 128 and X1/9 seats are interchangeable and I have a cinquecento sporting lying around and could see if they slide straight in if you want as they are nice bucketish comfortable seats.