[freeandfreezing]

The engines are probably from an older generation of 747. In what seems like a clever cost reduction strategy Stratolaunch bought a couple of used 747s and scavenged the engines, landing gear, etc. from them

Source

[Moonman62]

Wiki says they’re from a 747-400. Not bad but not the most modern either. After reading the Wiki on this project, I’d say it’s doomed to failure.