Just replace all Object occurrences with PlacedMorph:
PlacedMorph subclass: #SpaceWar
instanceVariableNames: 'centralStar ships torpedoes'
classVariableNames: ''
poolDictionaries: ''
category: 'Spacewar!'
PlacedMorph subclass: #CentralStar
instanceVariableNames: 'mass'
classVariableNames: ''
poolDictionaries: ''
category: 'Spacewar!'
PlacedMorph subclass: #SpaceShip
instanceVariableNames: 'name position heading velocity
fuel torpedoes mass acceleration'
classVariableNames: ''
poolDictionaries: ''
category: 'Spacewar!'
Mobile subclass: #SpaceShip instanceVariableNames: 'name fuel torpedoes' classVariableNames: '' poolDictionaries: '' category: 'Spacewar!' Mobile subclass: #Torpedo instanceVariableNames: 'lifeSpan' classVariableNames: '' poolDictionaries: '' category: 'Spacewar!'