If there is a rigger in your harbor/area that you feel comfortable with, I'd invite him/her to the boat for an hour of his/her time. This should be really, really simple, unless the previous owner tried to remove all traces of the staysail stay.
Briefly, you'll need:
A tang on the mast
for the upper end.
A chainplate (or heavy padeye) on the deck
for the lower end.
exiting below the mast tang that runs to the base of the mast, and perhaps back to the cockpit.
Tracks and sheet blocks on the cabin
top (I think).
A pair of small winches to tension the sheets
I think that you should be able to self-survey and see how many of these items are on the boat, but a rigger would figure it out in minutes.