It's not plug + play, but it will work making some adjustments to the frame in the cabinet that holds the metal frame of the monitor; thankfully nearly all old cabinets are made of wood.

BTW: The american 19" usually means 20" in Europe (different measuring), so there's only one inch difference to 21". I've just transplanted a 21"er into a System 1 cabinet, it was no big deal and you won't recognize it after installing the bezel.