126s produced before April 81 only need belts in the front that restrain the upper part of the body,
126s produced between April 81 and April 87 need a 3 point belt in the front.
126s produced after April 87 need a 3 point belt in the front and at least lap-belts in the rear.
Head restraints don't seem to appear on the MOT manual. I wouldn't worry about it, I have a 88 UK model with no head restraints and it passes.