If you want to cosplay as Ippo or any characters in that manga like Mashiba, just draw the abs and chest muscles, atleast. Use hena tattoo! And make it look aesthetically believable. A real artist could do that with proper shadowing techniques with body art painting. For example: Mashiba, you could just find a thin slender tall guy with long arms and no real muscles and body paint his face, add the thick eyebrows, and the wig, and the muscle body paint and the gloves and you got yourself a real-life replica.
If you want to cosplay as Ippo just draw the abs and chest muscles atleast. Use hena tattoo! And make it look aesthetically believable. A real artist could do that with proper shadowing technique with body painting.