Really surprised over so many posts, nobody -- 'specially from New England -- has chimed in with Willie Pep's name. He could do what he did at the height of career from the first day he stepped in to a gym.
In a way, yes...but you can't manufacture desire, heart, grit, determination and work ethic. Marciano had all these in spades. I think he's a good pick. I'd add the Manassa Mauler as well. Pacman from modern times. Guys like Johnny Tapia, Erik Morales and the like too.
Eckhard Dagge. That guy even loved to get punched, he let his opponents beat him up for the first half of his fights before he started to fight back.