[Jims5543]

Your first move would to be to hire a structural engineer to look at the construction and advise you if any load can be put on the trusses below.

Then go from there. I would imagine a good engineer could design a deck for you or suggest an architect that can do it.

You may also be able to get the original building plans from the local courthouse. It might take a few phone calls but they should be scanned onto microfilm. You should be able to purchase a copy.

While you there, pay a visit to the building dept and see if a permit / inspections will be needed.

Those would be my first steps before even talking to a contractor.