It's a shame...
It's unfortunate that Guillermo del Toro's version of Monster will likely never get made. If it does though, John Cho would make a great Tenma. I know he isn't of Japanese descent, but in Hollywood pretty much anyone who broadly fits a character's ethnic background is considered adequate.
Another actor who might work is Daniel Dae Kim. Granted, hypothetically, they might want to cast an actual Japanese actor.