This is one of the applications for which I think pressure treated lumber is well suited. Beware though that PT lumber doesn't hold paint well even after it has set long enough to dry out. If your jambs are entirely on the threshold and there is no rabbet, the procedure is similar, though you may have to cut the fasteners at the base of the jambs before being able to remove the old piece. Unfortunately the mortar sold for general use at lumberyards today is completely inappropriate for use on older brick structures. If used it can cause serious deterioration of the brick.
Modern mortar is too hard to use with older, softer brick.
A masonry wall needs to be able to accommodate the stresses it is subjected to from expansion, contraction, settling, etc. If the mortar is harder than the brick, these stresses will cause cracking and spalling of the brick itself. The other 20 percent can be portland cement for better adhesion and workability. The visual appearance of the mortar should closely resemble the appearance of the weathered existing mortar. This is accomplished by matching the materials that make up the mortar's aggregate as closely as possible, and applying the mortar in a way that imitates the existing mortar.