The following figure helps summarize most of the many mergers that have happened in the aerospace industry. However, it does not show the 2020 merger and split between UTC and Raytheon.
AC Delco: Originally were separated as AC Spark Plugs and Delco Electronics during Apollo era under GM later to be joined as ACDelco. Also helped provide parts for the lunar rover. Their website doesn't say anything about any aerospace projects but they have over 90,000 auto parts which many of them could be used in aerospace technology.
Aerojet-General: Merged with Rocketdyne, still in the rocket business.
Aeronca: Became a subsidiary of Fleet Aerospace in 1986, parent company reorganized as Magellan Aerospace in 1996. Still makes components for Boeing, Northrop Grumman, Lockheed and Airbus.
AiResearch: Yes, as Honeywell. Merged with Signal Oil & Gas in 1964 to become Signal, merged with Allied Corp. in 1985 to become Allied-Signal, purchased Honeywell in 1999 and adopted its name. Honeywell is a major manufacturer of aircraft and spacecraft parts.
Allison/Airite Products: As a subidiary of Sargent Industries, became the contractor for the Shuttle MPS surge propulsion receiver. Still in business as Sargent Aerospace.
AVCO: Still in business as the aerospace and defense subsidiary of Textron.
Beech Aircraft: Beech is also known as Beechcraft, it's still making airplanes as part of Textron.
Bell Aerospace: Was actually bought by Textron Corp. in 1960 and went on as Bell Aerospace Textron. Parent company of Bell Aerosystems and the only existing part of the company left, Bell Helicopter, which makes the Osprey, AH-1Z (aka Apache), and UH-1Y (aka Huey) for the military as well as plenty of civilian helicopters too.
Bell Aerosystems: Its parent company was Bell Aerospace Textron and was the division that developed the engines for the Agena A, B, D and Target Vehicle versions. Bell Helicopter is only surviving division of Bell Aerospace.
Boeing: One of the biggest aerospace manufacturers still existing, responsible for the 702 satellite bus, CST-100 spacecraft, and the SLS core stage.
Collins Radio: Purchased by Rockwell to become Rockwell Collins, got bought by United Technologies Corporation, still around as Collins Aerospace division of Raytheon.
Douglas Aircraft: Merged with McDonnell in 1967, merged with Boeing in 1997.
Elgin National Watch: Went out of business in 1968.
Grumman: Merged with Northrop, now the prime contractor for the James Webb Space Telescope.
Hamilton Standard: Merged with Sundstrand in 1999 to form Hamilton Sunstrand, merged with Goodrich in 2012 to form UTC Aerospace, part of the 2020 UTC-Raytheon merger and split. Still in business as part of the Collins Aerospace division of Raytheon, famous for airplane propellers but they make other components.
Honeywell: Yes. Acquired by Allied-Signal in 1999 but kept name. Honeywell is a major manufacturer of aircraft and spacecraft parts.
IBM: IBM Federal Systems Division was eventually sold to LORAL, and now part of Lockheed-Martin. Still around, of course, with vaguely described aerospace/defense activities.
International Latex (ILC Dover): Still making spacesuits, including the Extravehicular Mobility Unit used for Shuttle and ISS. Also made the airbags for Pathfinder, Spirit, and Opportunity, as well as blimps and airships.
Kollsman Instrument: Now a subsidiary of Israel-based defense company Elbit Systems. They are a subcontrator to Boeing.
Link: Per info from Link Trainer, via acquisitions from Hughes Training and Raytheon Systems, now the Link Simulation & Training division of L3 Technologies. They continue to make aerospace simulators.
Lockheed: Merged with Martin Marietta, now Lockheed Martin.
Marquardt: Split, sold, and resold; their bipropellant thrusters are now owned by Aerojet Rocketdyne.
MIT (Draper): Still working on many projects including aerospace, as of this writing the organization's web page has a section called "Draper to Advance NASA's Human Lander Mission".
North American Aviation: Partially blamed for the Apollo 1 fire, it merged with Rockwell later that year (1967). Built the Shuttle orbiters. Sold to Boeing in 1996 as Boeing North American, now part of Boeing Defense. (Rockwell is now part of Raytheon.)
Northrop: Merged with Grumman, now the prime contractor for the James Webb Space Telescope.
Nortronics: Was then the electronics division of Northrup, which merged with Grumman and has reorganized many times.
Pratt and Whitney: Pratt and Whitney's fuel cell division was founded in 1958, and produced the Apollo and Shuttle fuel cells. It became a subsidiary of United Technologies in 1985, was renamed UTC Fuel Cells, renamed UTC Power in 2001, sold to ClearEdge Power in 2013, went bankrupt in 2014, and is now part of Doosan Fuel Cell America. It no longer makes fuel cells for aerospace. (P&W's other divisions are now a subsidiary of Raytheon.)
Radiation Inc.: Merged with Harris Corp, still does aerospace work today, such as work with the GOES-R satellites.
Raytheon: Yes, still same name today, and builder of missiles, radars, and other aerospace components. Merged with United Technologies in 2020 and then split up; the two companies' aerospace divisions stayed as the new Raytheon, and their non-aerospace divisions became Carrier Corporation and Otis Elevator.
RCA: No. Spun off from General Electric in 1932, reacquired by GE in 1986, and dismantled. The division which made electronics for Apollo was sold to Thomson Consumer Electronics. RCA today exists only as a trademark for consumer products. GE is a significant defense contractor, but these involve divisions that had nothing to do with RCA.
Rocketdyne: Merged with Aerojet, still in the rocket business.
Simmonds Precision Products: Apparently became part of UTC Aerospace, but does not seem to be an active brand. If true, it would now be part of Raytheon.
Thiokol: Merged to form ATK, then Orbital ATK, and now is a part of Northrup Grumman, and still in the business. They still build solid rocket motors today.
Westinghouse Electric: The Westinghouse Electronics Systems Group was purchased in 1996 by Northrop Grumman to become Northrop Grumman Electronic Systems. Northrop Grumman reorganized in 2015 and 2020, making it difficult to ascertain exactly which of their divisions applies here.
