All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
Wearing the cross is forbidden, and it may lead its doer to disbelief as we have already clarified in fataawa 25725 and 114983. You should know that a Muslim is not automatically ruled to be a disbeliever (committing an act of disbelief) just by wearing the cross. The scholars have set conditions for declaring someone a disbeliever; in this regard, please refer to fataawa 8106 and 87963. This is in respect to the first question.
As regards the second question, there is nothing wrong with this project, and this is not creating or imitating the creation of Allaah; rather, it is using the mind and materials that Allaah has blessed the human being with in making this model brain, unless this is linked to a forbidden belief such as denying the Creator or claiming that the human being is able to create in the same manner that Allaah creates.
Also, making this brain is not included in forbidden images, as there is nothing wrong with making an image of only the head. Al-Buhooti said in Sharh Al-Iqnaa’, “...or making for it (the image) a head that is separate from its body, or images of heads without the body, then this is not disliked; as this is not included in the prohibition...”
Finally, it should be noted that the images made for the purpose of education, although they are of the kind that is prohibited, are excused because of the necessity.
Allaah knows best.