How can I compose a scene between Okonkwo and his father Unoka discussing the theme of power and masculinity in the novel Things Fall Apart by Chinua Achebe?

As you work on writing this scene, your primary focus should be the contrast in beliefs and actions of the two men and the way Okonkwo resents his father. You might want to frame the scene as a dream vision in which Okonkwo, shortly after his father's death, converses with Unoka's spirit in a dream. 
The beginning of the conversation might be Okonkwo talking about his lack of inheritance and how his father's debts and lack of title have made life difficult for his family even after his death. Next, Okonkwo should accuse his father of not being a good man and failing to live up to the standards of masculinity.
As a spirit, Unoka should display a sort of wisdom and self knowledge that he might not have had in life. He should admit that he lacked physical strength and the sort of assertiveness and aggressiveness admired in the warlike Igbo society. He should acknowledge that he is quite different from his son and that he understands his son's perspective. You might quote the following passage:

Even as a little boy [Okonkwo] had resented his father's failure and weakness.... And so Okonkwo was ruled by one passion-- to hate everything that his father Unoka had loved. One of those things was gentleness and another was idleness.

Nonetheless, Unoka might point out that his aversion to war and dislike of the sight of blood, although despised in his society, did not mean that he was a failure as a man, but rather that Igbo notions of masculinity, especially as understood by Okonkwo, are limiting.
Unoka might briefly play the flute in the scene and point out that his flute playing brought peace and contentment to himself and joy to his community. Although his buying wine and entertaining left him poor, it also brought happiness. He would argue that his lack of interest in material possessions, generosity, peacefulness, and dedication to bringing pleasure to the village even in the face of disapproval were a different kind of strength, and that Okonkwo's obsession with power and material goods was a weakness. 
Unoka might also point out that Igbo tradition emphasizes a balance between the masculine and feminine in each person and that it is precisely Okonkwo's rejection of the feminine that leads to his conflict with the Earth Goddess. He would argue that within each person, masculine and feminine need to be in balance. 
Another major difference between the two men was 

[…] he [Okonkwo] was not afraid of war. He was a man of action, a man of war. Unlike his father he could stand the look of blood. In Umuofia’s latest war he was the first to bring home a human head. 

In the scene you are writing, you could have Okonkwo boast of this. Unoka might respond by saying that it is this hypermasculinity and aggressiveness that lead Okonkwo eventually to harm his own family. If Unoka has in some sense failed by lacking the warrior spirit, Okonkwo fails by possessing it in too great measure. The dialogue could conclude with Unoka talking with the wisdom he has gained in the spirit world about the need for a balance within every person of the strong and the gentle, the artistic and the practical, happiness and striving, and masculine and feminine.