2018-2020 Blocks Flyers had a Euro bottom bracket; 2021s have a Mid. If the bearing threads into the frame it's Euro. If the bearings press directly in, it's Mid. I believe all had/have a 19mm spindle.
Depending on year, the brakes were either Promax or Tektro V-brakes, so any V-brake lever should work. They spec them with a "2-finger" lever, if that matters.
If you figure out the year model, the
SE Archives has many of the specs for each model. Look it up under the "Rad Series" of each year.