The original Meguro was 33 years old and Hayami found him when he was 13, that still leaves 20 years to have a kid and let's face it, the original was crazy enough he could have gotten the kid even in highly disturbing and illegal ways...even before he was found by Hayami.
But i am going with him becoming a father after he was 13, and Hayami adopting the kid and raising him to be a fighter(hence the muscles) in order to possibly get a saner version of Meguro in the future.
Let's face it, makes more sense than a clone and would explain why Masaki is calling Hayami his "dad", besides, considering how he didn't even hurt his foe in his first Kengan match and doesn't seem unhappy about it while being seemingly much saner, i am betting that Masaki isn't much like his biological dad.
But yeah, i am going with him not being a clone, but Meguro's kid instead and Hayami just taking advantage of it.