Well if you read my previous post about me improvising Shippuuden ending 8: BAcchikoi! on piano the you'll know what I'm talking about. The song was going great even though the left hand took quite a while to exactly get an idea of what would go good with the melody. Now the problem comes with the very first singing verse that goes like this:

"Oh my god! Oh my god!
Kinou no shippai de kujiketatte?
"
The Oh my god! Oh my god! Part took a LONG TIME to figure out that it was Eb but since that song isn't piano it's very hard to improvise this part so I thought I could just repeat and Eb octave like 3 times:

Oh my god! Oh my god!
Eb Eb Eb Eb Eb Eb That was just terrible so I thought about another improvisation method were you take a part of a song and change the rhythm
to almost completely different for just that one part of the song:

Oh my god! Oh my god!

eb eb c# g# g# f# with a slightly different rhythm than the real song.

Is this okay or should I not do this?

You gotta use your imagination there, rap songs are impossible to notate down unless you can find a small relative pitch in their voice.