Actually, I don't think the seat from a Thunderbird Sport will fit the Adventurer... they have different rear sub-frames.
There isn't much after-market for these bikes... some people have adapted the TB sport 'solo' seat for the Thunderbird, other then that you'll have to find or fab something to fit. Ditto for lights, blinkers, etc. Mecatwin in France (
www.mecatwin.com) makes an undertail for the Thunderbird Sport but again, don't think it will fit and Adventurer w/o serious modifications.
Do some searching around this forum, be sure the check out the 'modified classics' sticky up a the top - that will give you an idea of what people have done and how they've done it.