Elon Musk plans to assume the role of chief executive officer at Twitter Inc. after completing his $44 billion acquisition, taking the helm of the social media giant on top of leading Tesla Inc. and SpaceX. Musk intends to replace Parag Agrawal, who was fired along with other major executives upon completion of the takeover, a person familiar with the matter said, asking not to be identified discussing internal deliberations. The billionaire is expected to remain CEO in the interim but may eventually cede the role in the longer term, the person added. Twitter representatives declined to comment.