Options for restoring or otherwise utilizing old chimney
I would really suggest that you should hire a local expert to come out to your place to give you advice on the chimney condition, design and safety. I am quite sure that there was good reason to cap off and close up the chimney and fireplace in the past and it is highly unlikely that you will come to an understanding of the reasons without some seasoned advice. 

Some things to consider:


There was a time when many chimneys were built by laying in the bricks with damp sand that was mixed with lime and horse hair instead of regular cement bearing mortar. Aging by wind and rain could wash out this material in the upper part above the roof line leaving the bricks with open spaces from inside to outside.
Many old chimneys were built simply as a stack of bricks without any specialized flue liner at all. A good quality modern chimney would have a decent flue liner that was mortared in with a high temperature silica material that does not degrade with heat.
It is an art to design and build a chimney installation that will function well with a fire place. It is really impossible for anyone to evaluate your installation without being there first hand to attest to the the quality and design. 


Advice on whether to use some type of natural gas fireplace insert with using the existing chimney as a holder for a vent stack will depend on several factors that cannot be ascertained without direct evaluation. Structural safety of the chimney would be one issue that I can think of. Another would be whether the chimney opening can accept the insertion of a vent stack of sufficient size (diameter). Yet another is the uncertainty of maintaining the joint integrity of the vent stack as it is assembled piece by piece, up to 40 feet total, and lowered down into the chimney from above.