@Kekkaishi unfortunately the torrent is dead https://www.gaytor.rent/details.php?id=583261701ff122aa0d3a52bb6e8b689320d31b551d833b2d

I found it! It was GAMS676 Sex Man! Phew, finally I can sleep in peace.

@Kekkaishi lol, I was trying to find the word but "henshin" was all I can think of. Thanks for the recommendation, I'll look it up!
Btw, I recall the guy in the video that I watch found a mask in a locker room and when he uses that mask he turns into that muscular men.

A long time ago I downloaded a Japanese video in which the actor is skinny person but then henshin into a muscular masked men, he does bottom. Anyone knows which video is it?