Disk brakes from a 122 Volvo will fit on with minor modification, but don't really work much better than the drums. I'd only consider it if you are looking at new drums anyway - otherwise the wheel cylinders and shoes are much cheaper than calipers and pads. I've got the drums on mine and they work perfectly fine - just as good as I can recall the brakes working on my old front-disc equipped 122 sedan.

Installing them is a matter of taking the donor 122 down to the bare spindle - rotor & hub, caliper, dust shield, and caliper mounting plate. Then take the 544 down to the bare spindle as well - removing the drum/hub, wheel cylinder and backing plate. Then the 122 parts fit onto the 544 spindle relatively straightforward. Some considerations - The shock will interfere with the caliper - so either swap calipers left to right and mount them to the front (check for clearance with the sway bar) or swap the shock to the front of the A-Arms (they mount on removable bolts - reverse the bolts). You may need to drill a hole or two in the caliper mounting plate to match up to the holes in the 544 spindle. Hook up the flexible lines (might need some re-routing depending on the caliper's location - and new flexible lines are a great idea), bleed and voila' - you have front discs on your 544.