I wonder if they're the good English equivalent that would get the same effect across, I can't really think of anything
Dont think there are any in this specific case. At least not to the same degree of double meaning as "Let's go!"/"I'm going!" and "I'm cumming." The closest would maybe be "I'm cumming (for you - the enemy") but that doesnt really work. Sounds funny though. I guess you could technically write "I'm coming ~~~" maybe with some wrong letters to indicate slurred speech and people would get the double meaning in English, too. But maybe "I'm coming" implies more that you're responding to someone calling you over rather than that you're ready to go "plus ultra" lol
In Japanese, the "iku" for "going" and "cumming" sound the exact same and can easily be written the same way, since I guess the slang derives from the word "going" (which is true for many languages, I think English, too, though we have the alternate spelling of "cumming"). In the raws, it's using slurred speech and katakana, which is often used to emphasize words in manga. There are many ways to write "iku" where it could have either meaning, using pure hiragana or pure katakana or even with the formal kanji for "going" 行く - if there is also context indicating that someone is having an orgasm.