You find out by studying what other athletes with a plant based diet eat.

Here are a few to get you started

What Frank Medrano eats (vegan calisthenics athlete)

https://www.youtube.com/watch?v=BQ9sfVsBbS8

What Patrick Baboumian eats (vegan strongman)

https://www.youtube.com/watch?v=lz9H7MMuKjY

Body building advice - plant based diet

https://www.youtube.com/watch?v=Lr4LIOq3grw

And there are plenty of vegan or vegetarian body builder communities out there, like this one

http://www.veganbodybuilding.com/