Hello,
Restoration on a 1069 105 series 1 underway. I am amazed at the weigh of the original front seats. I assume its mostly due to the frame, are there similar looking style seats but much lighter. Mine have the adjustable head rest and side arms that wrap your sides.
There are, however, with lightness comes a heavy $ cost, I have done come research and the only ones similar to S1 seats are Ti Super replica ones, out of South Africa. Pm me if you want some further info.
Jim
GTA Junior came with seats that are similar to S1 seats in appearance but much more comfortable and much much lighter.
Being a limited series item, it may be difficult to locate and expensive.
I am not aware of any replicas.