Simplify the following text for a Grade 8 native English-speaking person.
Original: After World War II , V12 engines became generally obsolete in aircraft due to the introduction of turbojet and turboprop engines , which had more power for their weight , and fewer complications in large aircraft .
Simplified: After World War II , V12 engines were mostly replaced by turbojet and turboprop engines . These engines produced more power for their weight , and fewer problems in large aircraft .