Don't just look at stresses, there are languages with long and short vowels. Czech, Slovak and Hungarian uses: banán, Latvian uses: banāns.
So to answer: I would copy that and make the first short and the second syllable long.
This is following up on Rafael's comment.
The Romans did not have bananas, but the botanical name musa occurs in Latin since at least the 13th century, originally as a transcription of Arabic mawza in the Latin translation of Avicenna’s “Canon”. Later, it was adopted by Linnaeus as the (still current) botanical name for the genus that includes bananas. So ...