Check the bottom of the frame for corrosion.
The engine will need a complete rebuild.
I would only pay $600. because every part will need attention and
it is 40 years of corrosion and neglect(sitting).
You could easily spend $5000.00
Frame, Engine, brakes, New front brake hose.
New chain and sprockets(must do).
Seat ,exhausts etc.etc.,.
On a Roadstar Adventure.