Below is a transcription of the poem on his self-portrait, followed by my English translation:

花依清香愛

人以仁義栄

幽囚何可恥

只有赤心明

A flower is cherished for its pure fragrance./A man glories in humanity and justice./Imprisonment brings no shame,/As long as one’s heart is sincere.

[Takéchi Hanpeita’s self-portrait appears in Samurai Assassins, courtesy of Kochi Prefectural Museum of History.]

Takéchi Hanpeita is the focus of Part II of Samurai Assassins. Also see this recent post.