Good point from Clanad. There may be remains of the original mortar in there. Trying to knock it off would probably dislodge even more bricks.
If so, lay bricks in place "dry". No mortar. Hold all the bricks in place with simple timber wedges. A bit tricky, but quite do-able.
Point around the wedges with new mortar. Poke it into the joints.
Next day, remove wedges and finish with normal face pointing.