rob - if you are serious then i suggest picking up a standard seat cowl used (they don't fetch fortunes) and setting to work with a pair of scissors and some cardboard, make a tamplate of what it is you want and fibreglass it up. they'll be some work with brackets to fix the front seat to the front of the subframe and the pillion pad might have to be based upon the standard seat base to make fitting to the standard key release catch relatively straight forward, then it's off to the painters and trimmers....
otherwise scrappy watching for a gen 2 back end, but then you'll be into rear light swaps as well as issues lining up the seat catch and front mounts.
alternatively you could be cutting off your subframe and welding on another...
how much time, space, skills and motivation do you really have towards this?