Other than starting from scratch, there is currently no way to re-play boss fights. I'd like to be able to engage bosses again in some form; by revisiting their levels, for example. I missed out on the true ending by not having all collectibles when I beat the final boss, and now that I do, without being able to play the final boss again (properly, anyway; it kinda shows up but I'm not able to complete the fight anymore) I can't get the true ending without replaying the game from zero, something that I don't hate doing, mind you, since I had so much fun that I am likely to play the whole thing over anyway, but it'd be nice to have the option anyway.