Recognize an integral grate
The Heatilator RB36AI and RB42AI manual lists the integral grate with the fireplace, chimney and roof termination. It specifies GR1 or GR2 replacement grates for the models covered. Those part numbers illustrate how specific a replacement can be. Do not apply them to a different unit, even if its firebox looks almost identical.
Record more than the width
When everything is cold, photograph the fireplace front and the grate from outside the opening. Note the model, what has broken, and whether the old grate appears attached or freestanding. Leave bolts, retainers and panels in place. A clear picture of a broken support is more useful than flattening or bending it to get a measurement.
Keep a damaged grate out of use
The screen and glass should not be relied on to hold a load of rolling logs. Avoid building a temporary support from brick or using the firebox floor in place of a required grate. Ask for the approved replacement reference, how it will be fitted and whether the surrounding hearth or screen also needs inspection.