From the Lockheed Martin website
"Every so often employees at Lockheed Martin Aeronautics are asked to clean house. They go through their desk drawers and file cabinets to discard old and unnecessary materials to make space for the new.
This process often results in unearthing items of historical significance. This was the case in 1990 and again 2006, when concepts or advanced design drawings, reports, and other documents dating back to the 1940s and ’50s were found.
When the Lockheed facility in Burbank, California, was shutting down in the early 1990s, all the commercial aircraft data for aircraft such as the Constellation, L-188 Electra, and L-1011 TriStar was moved to Marietta, Georgia.
The file cabinets of data were placed in a warehouse for distribution to various groups. The Field Service group (now known as Technical Operations) inherited a large number of cabinets, including those for the L-188. While searching through the contents of those cabinets, three ring binders were found that were chock full of preliminary aircraft designs numbered CL-xxx, which stood for California-Lockheed.
Bill Slayton, a Burbank customer support employee, realized the historical importance of data available to him through the Advanced Design group. Slayton gathered descriptions, three-view drawings and isometric drawings and compiled these into a remarkable collection of the Lockheed advanced designs.
Slayton stored the data in the L-188 cabinets, most likely because the cabinets happened to have an empty drawer. Most of the CL-xxx collection has never seen the light of day."
Seems like a good place to make enquiries for a start. They will certainly have had an interest in the accident report.
Even if the documents are no longer with Lockheed Martin, they are likely to have gone to a university, aviation library, or some other organisation or individual with a particular interest in the history of avaiation. They should certainly be able to provide you with some clues as to where to continue your searches.