That's a plane designed in the 1960s and last produced in 1984. There's a massive gap in development which Lockheed would need to run through. Why would Lockheed even do this? (assuming there isn't a massive subsidy from US government).

Building a passenger jet isn't even that difficult. The difficult part is making it economical - low weight, fuel consumption, high reliability, parts availability, service/repair centers, training facilities, trained crews ...