The usual problem with both these brake arsm is that they have been overtightened in the past. Getting the rear one on its often necessary to remove the wheel and brake plate so it can be tapped on without knocking the spindle through. Probably best to get the other end sorted first so you know what angle is needed at the hub end.
Filing the serrations will probably help at the front and I wul dalso dress the serrations on the shaft itself to get a lead-in. If you take the silencer off (easy enough) you can use some 8 or 10mm threaded bar to draw the two parts together. Don’t forget to replace the spring first – its a PITA to get on afterwards but it can be left loose then use a short screwdriver to hook it into position once the arm is located.
Hope that helps
Peter