Hmm... recently on Wikia we added in Cheating category "how to add Items"

$game_party.gain_item( $data_items[101], 1 )

If You type that ( and i grabbed right item ( 101 = itemQuestHeadQuMilo6 ) ) You will get Adam's Head. I don't like that quest line but If i remember correctly his head drops on ground like regular drop item either u kill him or talk to him... unless there is bug in last versions... but I think You just missed it...

If You quit and - re-enter map head is no longer there.

Spawning it is only way, but You can just go to Milo without Adam's Head and return quest... I remember that Milo will pay lona and give XP but I dont remember if thats not breaking next quests...