Would you give any fighters pre 1920 a good chance at being champion today?

Provided that they get access to modern training and so forth I don't see why not. If you hypothetically plucked them out of their time and stuck them in a ring against modern fighters I think probably not. It's kind of taking a 1920s chess player and sticking them in there with Carlssen.

 

At the lower weight classes fighters are generally better today(90s-present) then in the past. The 1910s are a very severe exception to this and would torch everyone.