My name's Kel.
You could drill into either the mortar or brick.
Given the old mortar was probably fairly lean -- meaning it didn't use a lot of Portland cement -- and given its age it'll tend to crumble --
and given the new work isn't very thick --
thinking it through --
I'd install the anchor into the brick.
It'll provide a more solid connection.
If the mounting bracket has places for four screws I'd use plastic anchors.
If only two holes for screws I'd use metal.
The metal anchors exert more force on the brick.
Make sure to select anchors according to the weight supported.
The package holding the anchors will also specify which size hole to drill.
Too small a hole and you'll damage the anchor.
Too large a hole and it won't grab.
If one person drills and another person holds a vacuum cleaner hose close to the bit you can collect the dust as you go.
Have I answered your question?