The pillion seat..in either early or late form, is fast becoming one of the hardest parts to source and they are frequently missing. I get enquiries about them regularly.
The dear old Indians were making a copy of the early type which, though not perfect, could be easily modified to 'pass muster' as an original.
For some reason these do not appear to be available at the moment, though I have not trawled the Indian websites to find them if they are still made...that might be worth doing.
A good replica is needed for sure but the fact there are two different types complicates matters...and they would be expensive...there is an awful lot of work in them..main springs, mattress springs, a padded cover and clips, a complicated eleven piece, pivoting frame of rivetted up construction and a ribbed base with four rivetted brackets...Ian