Also from an romanization perspective, kenpo in Japanese is けんぽう in hiragana, 拳法 in kanji. https://jisho.org/word/拳法
Where I'm going w/ this language lesson is the alphabetical, aka hiragana, spelling breaks down as such:
け = ke ん = n ぽ = po う = u (or in this case making the po a longer sound)
There is no standalone "m" sound in Japanese, so if we are taking the name Kenpo from the Japanese pronunciation/reading, then Kenpo is the correct romanization technically if you want to go purist...
... however if you want to apply old traditional Hepburn romanization, then per Wikipedia "Syllabic
n (ん) is written as
n before consonants, but as
m before labial consonants:
b,
m, and
p." But Modern Hepburn doesn't use this, and still uses "n" and no "m" for the reading.
Hepburn romanization - Wikipedia
I majored in Japanese Studies for my BA/MA and I'm at a conversational fluency level (N3 of the 日本語能力試験) for background into my analysis of the above
Meanwhile, those same characters in Chinese are pronounced: Quánfǎ <-- nothing like the romanization of the Japanese into Kenpo/Kempo
So the TL : DR Kenpo and Kempo are both technically the same word, just different romanizations of the same word in Japanese.
Some masters liked 1 versus the other, so how we choose to use them is another matter.