For example could Matthew Saad Muhammad bulk up to 200 pounds and beat Larry Holmes in 1985/86? Or maybe Yaqui Lopez?
Not impossible but I wouldn’t pick any of them to. Michael Spinks was one of the greatest light heavy’s of all time with an awkward style and good blend of durability and power. Also had a bigger frame for a light heavy. He had the right set of tools to capitalize against an aging ( and complacent ) Holmes. Bob Foster couldn’t do anything at heavyweight despite all the times he tried. Saad didn’t have the chin or the right style. Eddie Mustafa and Dwight Qawi were both extremely tough but wouldn’t cut it.
I don't believe any other light heavyweight in history other than maybe Billy Conn, or Virgil Hill would beat Holmes. Spinks had the perfect combination of speed, awkward style, size, and perfect timing to pull off the upset of past prime Larry Holmes. No other Lt. Heavyweight in history had that perfect combination of attributes to beat that version of Holmes.