BMX steerer tube is pretty normal externally but with a thicker wall. The headset is designed for a 21.1 mm stem, not a 22.2 mm. You can replace or modify the locknut to allow fitting a 22.2 mm stem. Watch the crown race size if you swap forks. The difference between 27.0 and 26.4 mm can be a pain.
Most BMX cruisers were designed around 24" wheels, not 26". I guess you are talking about '50s Schwinns and suchlike with 26" wheels though.