[CivBase]

Lockheed Martin, Raytheon Technologies, General Electric, Northrop Grumman, and Honeywell all spring to mind. Most of those companies do not presently manufacture entire aircraft, but they're all in a position to do so if they acquired only a fraction of Boeing's assets. Raytheon Technologies in particular already makes engines, avionics, and interior systems. All they'd need is a manufacturing plant to assemble everything onto a fuselage.

Boeing and Lockheed Martin get a lot of name recognition for handling the high-level design and manufacturing of aircraft. But in reality, their planes are an amalgamation of engines, avionics, and interior systems which are often contracted out to other companies like those I listed. Many of them could step up and fill Boeing's shoes if a good deal presents itself - especially if there was extra incentive seeing as it would be a matter of national interest.