The "plugs" in the intakes, outlets and other places where the airplane can ingest junk are called "Foreign Object Debris", or FOD covers. FOD management is a major concern.
They can be soft, like tarps, or molded plastic to fit into specific locations. Their availability for models is fairly random, easier to find for more common aircraft.
"Remove Before Flight" pins or covers, the visible part of which is a red ribbon with copy on it, are not hard to find. Eduard makes nice ones.
My buddy Airman Jim was in the radio shop at Barksdale using an electric pencil to engrave his name on his tools. The Seargent came in.
"Hey boy, watcha doin" there?".
"Putting my name on my tools, sir".
"Oh, so when they find it in the ruins of a million dollar engine, they'll know who to return it to?".